* Undying can use Decay not only to reduce Sven's maximum health, but also to reduce the damage he can deal with his ultimate.
* Zombies spawned by Tombstone together with Flesh Golem slow Sven down significantly. Landing Sven's powerful but somewhat slow right-click on Tombstone would be a waste.

* Primal Roar takes Sven out of the fight completely, even with Black King Bar.

* Undying's Tombstone will cheerfully chew through treants, incidentally netting you some extra income during fights.
* The above interaction tends to ensure that skirmishes are heavily populated with units, thereby buffing Soul Rip

* Primal Roar and the slow from Boars make kiting Tiny very easy.

* Undying's Decay is a dangerous ability against Bristleback, as it makes him more vulnerable and reduces his tankiness.
* Undying's Tombstone together with Flesh Golem can slow Bristleback down significantly.

* Spectre must chase for kills and relies on movement speed for mobility in fights, and has no way to counter the Tombstone zombies.
* The potency of Soul Rip can be increased from nearby illusions from Haunt.
* Decay will shut down Spectre during the laning phase by effectively cutting down her initial health pool.
Additional:
* Spectre must chase for kills and relies on movement speed for mobility in fights, and has no way to counter the Tombstone zombies.
* The potency of Soul Rip can be increased from nearby illusions, so Undying can use Haunt against Spectre.
* Early game Decay will make Spectre easy to shut down in lane.

* Decay's stacking strength reduction makes Axe much less dominant in lane.
* Tombstone's zombies take no damage from Counter Helix, and will continuously put Axe's Blink Dagger on cooldown until dealt with.

* Decay, Soul Rip and Flesh Golem all deal magical damage which ignores Bulwark.
* Decay reduces Mars' natural strength and therefore his health pool, which is what he relies on to trade effectively in lane.

* Call of the Wild Hawk can scout Techies as they are placing mines.
* Call of the Wild Boar can clear Proximity Mines.
* The stun of Primal Roar will usually be enough to deal with the squishy Techies before they can try to suicide with Blast Off!.

* Primal Roar pierces Rage and can disable Lifestealer in fights.

* Decay can be good against Centaur Warrunner, as reducing the health of Centaur can make Double Edge risky.
* His zombies do not proc Return.

* Inner Beast helps Beastmaster's team destroy Supernova quickly.

* The Hawk can easily find Monkey King while he is sitting on a tree.
* Wild Axes can knock out Monkey King from tree.
* Boars drastically reduce Monkey King's attack speed, making it harder for him to gain Jingu Mastery stacks.
Additional:
* Wild Axes can chop down trees, making it difficult for Monkey King to utilize Tree Dance well.
* Call of the Wild Hawk can scout Monkey King when he is perched on top of trees.

* Attacks from Boars and the attack speed boost from Inner Beast will quickly wear down Refraction.
* True Sight provided by Level 3 Necronomicon can see Templar Assassin in Meld.

* Due to Spirit Breaker's low attack speed, Tombstone is problematic for Spirit Breaker.
* The zombies summoned from Tombstone apply a stacking movement speed slow, which can significantly reduce Greater Bash's damage.
* Soul Rip and Decay allows Undying and his allies to survive from incoming ganks of Spirit Breaker.

* Undying saps strength from heroes with Decay, and Ogre Magi's high strength gain will allow Undying to steal more and more strength, becoming stronger with each use.
* Due to the Strength steal from Decay, Undying is normally very tanky, and has high health, making it harder for Ogre Magi to kill him.

* Necronomicon Warrior's Mana Break can deplete Wraith King's mana and prevent him from using Reincarnation.
* Wraith King's lack of area of effect spells makes him easily overwhelmed by Beastmaster's summons.

* Primal Roar and the slow from Boars makes kiting Ursa very easy.

* Beastmaster's Hawk can find Tinker easily while he is pushing or blinking into trees.

* Primal Roar allows Beastmaster to keep Lycan in place after he used his Shapeshift.

* Heroes with a slow attack rate will find themselves struggling to take out Tombstone, like Doom.

* Heroes with disables that ignore spell immunity will render Phantom Assassin quite useless in teamfights throughout the game: Enigma, Vengeful Spirit, Doom, Bane, Beastmaster, Pudge.

* Heroes who relies on illusions and summons (Beastmaster, Chaos Knight, etc.).

* Anti-Tank heroes like: Razor, Necrophos, Undying.

* Heroes who rely on high mobility are especially vulnerable to Primal Roar: Storm Spirit, Ember Spirit, Queen of Pain.

* Generally, any fragile heroes will have a hard time dealing with Undying's tankiness, and even more so if Decay is used on them. Some examples are Crystal Maiden and Witch Doctor.

* Heroes who rely on high mobility are especially vulnerable to Primal Roar: Storm Spirit, Ember Spirit, Queen of Pain.

* Illusion or summon-based heroes can be a problem to Viper due to his lack of AoE damage: Naga Siren, Phantom Lancer, Beastmaster

* 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 with disables that go through spell immunity can end Bloodseeker before he has the chance to do anything: Batrider, Beastmaster

* Heroes with illusions and summons can manfight Night Stalker: Terrorblade, Beastmaster, Broodmother.

* Enigma's left level 25 talent (increasing the Eidolon count from three to eleven) powers up Undying's Soul Rip.

* Because only one clone needs to be killed in order to kill Meepo, heroes that excel at singling out enemies can also be a detriment to him, such as Batrider, Beastmaster, Pudge and Clockwerk.

* Heroes who rely on high mobility are especially vulnerable to Primal Roar: Storm Spirit, Ember Spirit, Queen of Pain.

* Winter Wyvern cannot handle the Zombies created by Undying's Tombstone very well. In most situations, Zombies will have more movement speed than her.
* Undying can gain one or two easy Decay stacks on an ally in Cold Embrace. The ally will still take the damage as well.
Additional:
* Winter Wyvern cannot handle the Zombies created by Tombstone very well. In most situations, Zombies will have more movement speed than her.
* Undying can gain one or two easy Decay stacks on an ally in Cold Embrace. The ally will still take the damage as well, since it is magical.

* Heroes with other kind of disables that pierce spell immunity can hold Pangolier in place during Rolling Thunder: Axe, Beastmaster, Enigma, Magnus, Pudge, Batrider, Bane, Winter Wyvern

* Primal Roar and the slow from Boars make kiting Troll Warlord very easy.