* Nethertoxin disables Desolate and Dispersion, leaving Spectre with far less damage output and damage mitigation in engagements.
* Spectre is one of the weakest heroes at early levels. Viper can stifle her farm with early fights before Spectre purchases any major items.
Additional:
* Nethertoxin disable passives, negating Spectre's damage and survivability by removing both Dispersion and Desolate.
* Due to Haunt and Radiance's burn, Spectre is heavily susceptible to being persistently slowed and damaged by Corrosive Skin without even directly attacking Viper.
* Poison Attack and Viper Strike further slow Spectre's movement speed to a crawl, while his improved magic resistance means he can easily shrug off Radiance's burn.

* The movement speed granted from Shapeshift means that Spectre has no way to punish Lycan if he wants to splitpush or pick off supports. Furthermore, he will kite Haunt and cannot be slowed by Spectral Dagger.
* As Lycan typically has Summon Wolves out and builds Necronomicon, very rarely will he be damaged by Desolate if he does manage to get into a fight against her.
* Lycan is often part of an early game push strategy which is often the best counter to Spectre because she needs a lot of time and space to come online as a late game carry.

* Nethertoxin breaks Fury Swipes, making Viper a decent counter to Ursa in a laning stage.
* Viper Strike is a strong, undispellable slow, making it much easier to kite Ursa for everyone.

* Lycan's ultimate makes him immune to slows from Ursa's Earthshock as well as allowing him to outmaneuver Ursa during Enrage. Lycan can also easily chase down Ursa while shapeshifted, should Ursa try to flee.

* Corrosive Skin minimizes the damage Viper takes from all of Ancient Apparition's abilities.
* Ancient Apparition's slow movement speed, squishy nature, and lack of escape abilities can easily be taken advantage of through Poison Attack and Viper Strike.

* Shapeshift increases Lycan's movement speed and renders him unslowable, allowing him to easily avoid and escape Ancient Apparition's abilities.
* Lycan has many fast summons, both his wolves from Summon Wolves and from the frequently built Necronomicon, all of which can do heavy damage to Ancient Apparition due to his immobility and low armor.

* Corrosive Skin minimizes the damage from Impale and Mana Burn.
* Nyx Assassin cannot survive several poison damage-over-time, even Spiked Carapace is activated.
* However the Vendetta break can be dangerous for Viper.

* Lycan is a common Necronomicon 3 carrier, hindering his attempt of initiate or escape using Vendetta.
* And the Last Will can be killed quickly with Nyx Assassin's high burst damage, although he can at least partially counter it with a well-timed Spiked Carapace.
* Lycan's high HP easily shrug off Nyx Assassin's burst damage and fend him off.

* Corrosive Skin counters Zeus in lane by canceling the effects of Clarity as Zeus relies on Arc Lightning to farm and secure last hits.
* Corrosive Skin also increases Viper's magic resistance to shrug off Zeus's magical nukes.
* Nethertoxin breaks Static Field.
* Due to Zeus' poor mobility, Viper Strike will cripple Zeus and will allow Viper and his teammates to land a lot of attacks.
Additional:
* Corrosive Skin severely reduces Zeus's damage output throughout the game.
* During the laning stage, Viper can abuse Zeus' reliance on spells by hugging the creep wave. Zeus will take constant damage over time from Corrosive Skin unless he drives Viper from the lane or returns to farming in his absence.
* Prior to buying Blink Dagger or Force Staff, Zeus is fragile and possesses low mobility. Any of Viper's numerous slows can set up an easy kill.

* Zeus can deal a huge amount of damage, but he lacks defensive abilities, such as good mobility or resistance. Due to that, he doesn't like to be touched by any means. Heroes with higher damage output and good mobility will often get easily near Zeus and trade damage (eventually causing Zeus's downfall). Such Heroes include: Lycan, Slark, Storm Spirit, Spectre and others.

* Nethertoxin breaks Sniper's passives.
* Viper Strike together with Poison Attack can slow Sniper and tears through his low health and mobility.

* You can easily kill a squishy Sniper, due to your high movement speed and his lack of escape.

* Heroes with damage-increasing passive abilities, such as Viper and Riki, will usually kill Silencer in a short time before he even has time to set up his abilities.

* Shapeshift is useful for killing or escaping Silencer.
* Summon Wolves, and later Necronomicon units, can be used to hunt a badly wounded Silencer down - even when Lycan himself can't fight.

* Multiple attackers thanks to summoned units or clones: Meepo, Witch Doctor, Lycan, Enigma, Shadow Shaman
* Heroes that can quickly deal damage to her, regardless of how weak they are, can easily break through her Refraction:
* Multiple attackers thanks to summoned units or clones: Meepo, Witch Doctor, Lycan, Enigma, Shadow Shaman
* Multiple attackers thanks to illusions: Phantom Lancer, Chaos Knight, Naga Siren (bonus for her Ensnare). Also despite the visual effects, Templar Assassin does not spill damage against illusions or buildings, so she is not a really good hero to clean up illusions.
* Weak damage but high tick rate or persist in a long time: Slark, Gyrocopter, Queen of Pain, Shadow Shaman (Shackles), Pugna

* The damage over time from Viper's Poison Attack and Viper Strike rapidly wear down Refraction charges, allowing Templar Assassin to be damaged. Additionally, Corrosive Skin causes her attacks to apply damage over time on her as well.
* Nethertoxin is a big threat to Templar Assassin. Her initially low range making it very hard to deal against Nethertoxin in the laning phase. At any stage of the game, a Nethertoxin can always force her to undo her Meld.
Additional:
* All of Viper's abilities quickly burn through Refraction charges.
* Templar Assassin's short range, aside from Psi Blades' spill damage, makes her vulnerable to Poison Attack harassment.
* Templar Assassin traditionally forgoes health and armor items for damage. Outside of Black King Bar, Viper will almost always be able to kill her when Refraction is down.

* Heroes with low armor and little mobility, like Huskar, Io, etc. will also have an unfortunate fate.

* His Poison Attack is very useful against Huskar, since he tends to stay at low health.
* Nethertoxin disables passive skills like Berserker's Blood.
* It is dangerous for Huskar to jump on Viper because Corrosive Skin and Viper Strike with the slowdown of movement and attack speed which usually will be deadly for Huskar.
Additional:
* Nethertoxin disables Berserker's Blood, which Huskar relies upon for attack speed and healing.
* The damage from Burning Spear and Life Break is largely absorbed by Corrosive Skin. It also will return some damage, making it risky to use Burning Spears in the laning stage.
* Because Huskar wants to be on low health for Berserker's Blood, he is particularly vulnerable to Poison Attack.

* Illusion and heroes with summons are difficult to kill with Chen's single-target offensive spells: Terrorblade, Phantom Lancer, Chaos Knight, Broodmother, Naga Siren, Lycan, Visage. However, Chen can convert enemy summons with Holy Persuasion, as long as they don't bunch up.

* Hand of God has a long cooldown, so damage over time spells can be hazardous for Chen, forcing him to use it prematurely: Jakiro, Venomancer, Viper, Dazzle, Phoenix.

* The slow applied from Corrosive Skin offsets the movement speed bonus granted by Scorched Earth and the tick-based damage from Infernal Blade and Doom will continually slow Doom down. This allows Viper, much like Sniper, to 'kite' Doom and keep him at a safe distance between the two. Additionally, Corrosive Skin applies an attack-speed slow which is fairly crippling to a hero such as Doom that, naturally, has a high base attack-time, poor agility gain and builds little attack-speed items.
* Since Doom does not disable auto-cast abilities, Viper's Poison Attack will continue to work. Doom may find it hard to close the distance between himself and Viper, and thusly difficult to effectively lockdown and continually deal damage through his abilities.

* Most of Magnus' abilities except Empower will trigger damage through Corrosive Skin.
* Poison Attack and Viper Strike can slow Magnus during Skewer, as well as putting his Blink Dagger on cooldown, preventing him from initiating or re-initiating.

* Tiny has low movespeed and attack speed, making him vulnerable to Viper's slows.
* Corrosive Skin provides magic resistance to lower Tiny's magic burst damage.

* Nethertoxin disables Vengeance Aura.
* Vengeful Spirit has no mobility spells aside from Nether Swap, which Viper can take advantage of with his slows.
* Viper can easily bully Vengeful Spirit out of a lane with Poison Attack.
Additional:
* Nethertoxin prevents attack damage bonus from Vengeance Aura and the components of Nether Swap.
* Both Magic Missile and Wave of Terror are counteracted by Corrosive Skin, making Vengeful Spirit's fighting harder against Viper.

* Nethertoxin prevents the lifesteal from Ransack. The silence from Viper's level 25 talent also prevents Meepo escaping with Poof.
* Meepo and his clones have a lack of magic resistance, so they're vulnerable to Poison Attack & Viper Strike directed onto one clone.

* Nethertoxin prevents buildup of Fiery Soul stacks.
* Corrosive Skin provides magic resistance against Lina's spells.
* Poison Attack and Viper Strike can burn away Lina's small health pool.

* Nethertoxin breaks both Necromastery and Presence of the Dark Lord, reducing his damage massively.
* Corrosive Skin gives Viper bonus Magic Resistance against Shadowraze and Requiem of Souls.
* Viper can easily take advantage of Shadow Fiend's lack of escape and mobility spells with Viper Strike and Poison Attack. Poison Attack also serves as a great way to harass Shadow Fiend.

* Shapeshift allows Lycan to get close and disable Marksmanship.
* Shapeshift also makes him immune to slows like Drow Ranger's Frost Arrows.
Additional:
* The Wolves' speed causes the knockback from Gust to be ineffective.
* She lacks escapes and loses her Marksmanship when someone is near her. Lycan takes advantage of these weaknesses.

* Nethertoxin breaks the damage block from Bulwark.
* Poison Attack and Viper Strike's slows make it difficult for Mars to contribute to fights due to his low mobility.
Additional:
* Nethertoxin breaks Bulwark entirely, making Mars far less tanky. Using Nethertoxin inside Arena of Blood may discourage enemies from following up with Mars' initiation.
* Rod of Atos is currently common on Viper and keeps Mars in position long enough to be killed.
* Viper is not very tanky earlier in the game, so positioning against Mars is important since a well-timed Spear of Mars can be the end of Viper.

* Viper's Nethertoxin disables Cloak and Dagger, negating Riki's invisibility and much of his damage. It also makes him more vulnerable to magic damage, which does not go well with Riki's low health pool. With the talent applied, he can't even blink to safety or hide with Tricks of the Trade.
* Corrosive Skin makes it hard for Riki to chase Viper and attack him due to the attack speed slow.
* Viper Strike stays on Riki even after he goes invisible.
* Poison Attack makes it risky for Riki to get last hits and can make the laning phase difficult for him.

* Lycan doesn't care much about Lich's slows thanks to Shapeshift.
* Chain Frost can bounce to the wolves that Lycan summons.
Additional:
* Lich possesses a lot of slows, which Lycan doesn't care about when he Shapeshifts.
* Lich has no in-built escape, which makes him a good target for Lycan.

* Shapeshift allows Lycan to chase down Earth Spirit.

* Corrosive Skin minimizes the damage Viper takes from all of Disruptor's skills.

* Corrosive Skin means Viper can often outlast Venomancer in a battle of attrition.
* Venomancer has no mobility without items, which makes him vulnerable to constant slows.

* Corrosive Skin gives Viper additional resistance to Phoenix's magical damage, and since they deal damage over time, Phoenix would be constantly afflicted by his passive's own debuff, making him not an ideal target for Phoenix's harassment. Phoenix's abilities also constantly drain its own health away, becoming more vulnerable to Poison Attack.

* Nethertoxin prevents Heartstopper Aura from dealing damage or healing Necrophos.
* Viper's magic damage is amplified whenever Ghost Shroud is used.
* Corrosive Skin makes Viper tankier against Death Pulse and Reaper's Scythe.

* Nethertoxin prevents the buildup of Jingu Mastery stacks.
* Viper's poison magical damage-over-time stops Monkey King from using Tree Dance. It also ignores the extra armor bestowed by Wukong's Command.
Additional:
* Nethertoxin prevents the buildup of Jingu Mastery stacks.
* Monkey King's high base movement speed is of little worth against the array of slows Viper possesses.
* All of Viper's magical damage over time stops Monkey King from using Tree Dance. It also ignores the extra armor bestowed by Wukong's Command.

* Nethertoxin breaks Multicast.
* Corrosive Skin minimizes the damage during Ignite and Fireblast.

* Mirana's Leap is ineffective if Lycan uses Shapeshift. It also allows him to dodge Sacred Arrow.
* As a natural Necronomicon carrier, he can reveal Mirana and her allies hiding in Moonlight Shadow and the Necronomicon Warrior can kill Mirana from an unintentional Sacred Arrow or Starstorm upgraded with Aghanim's Scepter.

* Because Poison Attack has a low cooldown at all levels, Aphotic Shield is ineffective as a hard dispel.
* Nethertoxin prevents Borrowed Time from activating passively and blocks Curse of Avernus

* Void's slow is useless against Lycan by activating Shapeshift.
* Night Stalker's single target abilities (with the exception of Void upgraded by Aghanim's Scepter) is not very effective against Lycan's Summon Wolves and Necronomicon minions during the early and mid game.

* Viper's poison abilities can slow Bounty Hunter's movement speed and allows to chase him with True Sight items.
* Nethertoxin can break Jinada, and the silence from level 25 prevents Bounty Hunter from using all of his spells.

* Nethertoxin cuts off Gravekeeper's Cloak, Visage's primary defense mechanism.
* Viper can ignore Grave Chill's attack speed slow with his magic damage output.
Additional:
* Since Visage is a slow hero, Viper can easily kite him with Poison Attack and Viper Strike while successfully chipping away at Gravekeeper's Cloak.
* Nethertoxin removes all layers of Gravekeeper's Cloak, leaving Visage exposed.
* Corrosive Skin makes Viper highly resistant to Soul Assumption.

* Viper's Poison Attack and Viper Strike that can slow Razor, and is one of the few heroes that can keep him immobile.
* Altogether, Viper's skills reduce Razor's attack speed, heavily reducing the benefit of the damage drain from Static Link.
Additional:
* Storm Surge is counteracted by Nethertoxin, leaving Razor without faster MS.
* Static Link's damage drain doesn't bother Viper a lot as most of his early to mid game damage is magical.

* Lycan and Weaver has abilities that allow them to move at maximum movement speed, and those abilities can't be dispelled, potentially wasting the slow from Demonic Purge.
* You can potentially kill a fragile Weaver by using disruption during Shukuchi with a well-placed Sentry Ward, wasting the Shukuchi. But odds are he'll wait to get low health, Time Lapse the damage away and Shukuchi away once more.

* Heroes with summons like Broodmother,Enigma, Invoker, Nature's Prophet, Lycan, Visage;or illusions, like Naga Siren, Phantom Lancer, Terrorblade are effective at keeping the choke points and rune spawns safe from Proximity mines in the early game.

Additional:
* He can send wolves to defuse Proximity Mines, and he can catch up Techies and kill him before they can deny themselves. Even if the suicide is successful, Lycan can still survive because of his tankiness.

* Heroes who can easily harass Crystal Maiden out of lane in the early game (Clinkz, Viper) can prevent Crystal Maiden from being able to level effectively, and can ultimately slow down her level 6, 12, and 18 to a point where enemy carries already have items to address her ultimate.

* Skywrath mage also has a terrible time fighting against anything that isn't a hero. As Mystic Flare only affects heroes, his skills are mostly target-based and he has terrible strength and agility, he can easily be kited or harassed by illusion-based heroes like Chaos Knight and Phantom Lancer, or by Heroes that summon other units like Lone Druid, Lycan and Beastmaster.

* Heroes that can slow Charge of Darkness by a huge amount: Venomancer, Viper, Dazzle, Arc Warden

* Heroes with low armor and little mobility, like Huskar, Io, etc. will also have an unfortunate fate.

* Melee heroes with low mobility can be kited and killed by Viper: Treant Protector, Kunkka, Alchemist, Troll Warlord, Wraith King, Tiny, Sven.

* Viper can bully Alchemist out of mid lane quite easily, forcing him to either retreat into jungle, or stay harassed in lane, and get very few last hits. Vipers break also completely disables Greevil's greed which prevents Alchemist to earn his amazing amounts of money.

* Poison Attack and Viper Strike slows Axe significantly, and puts his Blink Dagger on cooldown, preventing him from initiating.
* Nethertoxin can break Counter Helix.
Additional:
* Nethertoxin prevents Counter Helix from triggering, which significantly lowers Axe's damage output in the early to mid game.
* Viper's various slows and damage over time prevents Axe from initiating as well as putting his Blink Dagger on cooldown.

* In the early game, a Dragon Knight without Black King Bar is basically defenseless against Viper.
Additional:
* Dragon Knight is easily harassed by Poison Attack after the laning stage ends.
* Corrosive Skin protects Viper from Breathe Fire and returns damage.
* Nethertoxin mitigates Dragon Blood, Dragon's Knight main source of armor and health regeneration.
* Between Poison Attack and Viper Strike, Viper can keep Dragon Knight permanently slowed even in Elder Dragon Form.

* Nethertoxin negates Essence Shift and the regeneration aspect of Shadow Dance.
* Dark Pact doesn't mean a lot against Viper's ubiquitous slows.
* Viper becomes a relevant presence in team fights before Slark can procure his standard items (e.g. Shadow Blade).

* Nethertoxin prevents Timbersaw from gaining stacks on Reactive Armor with silence talent also prevents Timbersaw using his abilities.

* Viper's skill set makes him an effective early game ganker against Anti-Mage before he is sufficiently farmed.
* Viper's Corrosive Skin and low mana pool causes him to take little damage from Mana Void.
* Nethertoxin disables Mana Break and Counterspell, weakening both his offense and defense.

* Poison Attack makes it hard for Void Spirit to lane against him.
* Corrosive Skin reduces the magical damage from Void Spirit's spells.

* Shapeshift allows Lycan to escape from Upheaval with 650 movement speed.
* Warlock doesn't have a lot of AoE abilities to deal with Lycan's wolves from Summon Wolves.

* A Lycan with Shapeshift can always run down Puck and kill him very quickly due to his low armor. It is important to abuse as many cliffs and trees as possible to escape from a Lycan.

* Magnetic Field is useless against melee heroes, so Lycan can attack Arc Warden without missing.
* Flux doesn't affect Lycan that much, because Shapeshift grants immunity to slows and because he is never alone.
* If their Tempest Double has been called, Lycan can still kill Arc Warden before the double expires.

* Heroes with haste can negate Enchant's slow: Weaver, Dark Seer, Lycan, Centaur Warrunner.

* Heroes with break can disable Untouchable for a while: Doom and Shadow Demon (via Aghanim's Scepter), Viper

* Heroes with single-target abilities will be overwhelmed by Lycan and his summons: Wraith King, Bane, Phantom Assassin.

* Melee heroes with low mobility can be kited and killed by Viper: Treant Protector, Kunkka, Alchemist, Troll Warlord, Wraith King, Tiny, Sven.

* Nethertoxin breaks Phantom Assassin's passive skills.
* Viper Strike goes through Black King Bar.

* Heroes with single-target abilities will be overwhelmed by Lycan and his summons: Wraith King, Bane, Phantom Assassin.

* Nethertoxin disables Bristleback's passive.
* Viper Strike and Poison Attack slow down Bristleback's movement speed so he's unable to get out from Nethertoxin.
Additional:
* Nethertoxin's break removes Bristleback and prevents Warpath from building stacks. Besides losing his eponymous defensive passive, Bristleback also lose his ability to build up damage over lengthy battles.
* Viper can perpetually kite Bristleback with Poison Attack and Viper Strike.

* Heroes who can harass Omniknight out of lane will keep him weak in the mid game: Viper, Venomancer, Clinkz

* Poison Attack and Viper Strike allowing to kite and slow Earthshaker significally, putting Blink Dagger on cooldown and preventing him from initiating or re-initiating.
* Nethertoxin breaks Aftershock. The silence from Level 25 talent also prevents Earthshaker from initiating or re-initiating.
Additional:
* Being a melee hero, Earthshaker's movement is liable to suppression by Poison Attack and Viper Strike.
* Earthshaker's kit, with the exception of Enchant Totem, deals magic damage. Corrosive Skin effectively blunts his killing potential with a standard Blink combo among other things.
* Nethertoxin disables Aftershock, which removes the stun and bonus damage from Enchant Totem and Echo Slam. Additionally the level 25 talent will silence Earthshaker, making him vulnerable to Viper's teammates without stuns.

* Corrosive Skin makes Viper receive less damage from Invoker's spells
* Viper can harass Invoker with his Poison Attack in the early game.
* Viper Strike and Poison Attack can kill Invoker easily if Ghost Walk is on cooldown.

* Nethertoxin cancels out Kraken Shell and stops the hard dispel from passively activating, preventing Tidehunter from functioning as a frontline tank.
* Anchor Smash is mostly irrelevant against a ranged hero.
* Tidehunter relies on the threat of Ravage, which has an obscenely long cooldown. Viper can exploit the downtime with his short cooldowns.

* Heroes with single-target abilities will be overwhelmed by Lycan and his summons: Wraith King, Bane, Phantom Assassin.