Tainted Apollyon is a character added in The Binding of Isaac: Repentance. He is the alternate version of Apollyon. Tainted Apollyon is unlocked by reaching Template:Stage and using either the Red Key or the Cracked Key to access a special room in one of the walls while playing as Apollyon.
Tainted Apollyon starts with 2 heart containers, Abyss in the consumable slot, and 1 red Abyss locust familiar. When Abyss is used, items in current room are destroyed and turned into further locust familiars, some items granting special types of locusts. They charge in the direction Tainted Apollyon is firing until they hit something, dealing Tainted Apollyon's damage per tick.
The randomness of Apollyon's Void is substituted for a guaranteed DPS upgrade with Abyss. This allows for easier judgement over whether active or mediocre items should be destroyed.
If Tainted Apollyon uses Abyss while picking up an active item, it will be turned into a locust; this is due to the change to items being able to be destroyed while being picked up in Repentance.
This does not apply to any other characters, as Tainted Apollyon is the only one who can have Abyss in his consumable slot, and therefore both pick up an active item and use Abyss simultaneously.
In rooms where there are multiple items to choose from, using Abyss will give only one locust for each item choice/set.
Libraries can be fully absorbed, making them useful to find.
Locusts deal full damage per tick, meaning that it is important to find a balance between the number of locusts spawned and amount of damage dealt. Items from the Boss item pool are usually good sources of damage, so it may be worth it to seek them out immediately.
Keep in mind that some Items give special Locusts. However, many of these Items are quite powerful, and may be more valuable if collected than if they are turned into Locusts.
Ipecac, for example, generates a unique poison locust with increased damage when Abyss consumes it; however, taking the item itself is almost always a superior choice, as the damage bonus from Ipecac affects all locusts Apollyon has, scaling further than the single locust would.
Like Void, Abyss can be used while holding an item, meaning it is completely possible to buy shop items and consume the item to grant a locust. One-time or low-use items such as Mom's Coin Purse, Booster Pack, and Voodoo Head can be great items to do this with.
This can also be used with blind item pedestals (such as on the alternate path or with Curse of the Blind), allowing Tainted Apollyon to use Abyss at the last second should he find the collected item to be undesirable.
Tainted Apollyon has very low Tears. Items such as Sad Onion and Torn Photo, if not absorbed, can be invaluable.
While Soy Milk gives a heavy tears upgrade, it will also decrease the damage of your locusts drastically. As such, it may be better to destroy it for a locust.
Extension Cord: Every locust will generate a beam of electricity, significantly increasing the range and damage potential of Tainted Apollyon.
Genesis will remove all locusts, and does not replace them with items, making this item quite weak for Tainted Apollyon.
Bugs
Bug!
When using Esau Jr., all Red Locusts are permanently lost, even after switching back.
Trivia
The Hebrew term Abaddon, and its Greek counterpart Apollyon appear in the Bible as a place of destruction and an archangel of the abyss. The supposed crack on Apollyon's face is a reference to that void that is used to reference the bottomless pit which is Sheol.
The crack on Apollyon's face could also be a reference to Azami Kurotani from the manga Uzumaki by Junji Ito, who has a very similar crack on her face.
The achievement for unlocking Tainted Apollyon is "The Empty", a possible title for the character.
Apollyon. Tainted Apollyon is unlocked by reaching Template:Stage and using either the 
Red Key or the 
Cracked Key to access a special room in one of the walls while playing as Apollyon.
Abyss in the consumable slot, and 1 red Abyss locust familiar. When Abyss is used, items in current room are destroyed and turned into further locust familiars, some items granting special types of locusts. They charge in the direction Tainted Apollyon is firing until they hit something, dealing Tainted Apollyon's damage per tick.
Apollyon's 
Bug!
Esau Jr., all Red Locusts are permanently lost, even after switching back.
Apollyon Unlocks 
Tainted Apollyon Unlocks 
Characters 
The Binding of Isaac: Rebirth
