The character is split into two parts, both existing at the same time. Tainted Soul resembles The Soul and is directly controlled, but cannot attack in any form except through the skeletal Tainted Forgotten. Tainted Soul cannot obtain red heart containers; it can only obtain soul, black, or bone hearts.
Tainted Forgotten is completely invulnerable, immobile, and does not have a health bar. Colliding with Tainted Forgotten will cause Tainted Soul to hold it over its head, and pressing any of the fire buttons will throw the skeletal body in a given direction.
Tainted Forgotten performs a bone swing attack while thrown for 3x damage, and deals an additional 5 points from body impact. While on the ground, Tainted Forgotten can continue swinging the bone or charge up for a bone throw attack in a similar manner to the standard Forgotten.
Tainted Forgotten can be thrown with 360 degrees of movement using an analog stick or mouse controls.
Enemies almost always target Tainted Soul alone, but Tainted Forgotten's body can also be used to block enemy projectiles and movement.
Both Tainted Forgotten and Tainted Soul can pick up items and consumables, unlock doors and chests, and pay for items (both in shops and devil deals). However, Tainted Forgotten will only do this if it is thrown into the object it is interacting with. Tainted Forgotten is automatically held by Tainted Soul upon entering a room. Bombs placed and consumables and trinkets dropped will spawn on the location of Tainted Forgotten only. Tainted Forgotten can be thrown through doors to enter adjacent rooms, and being invulnerable, curse room doors can be passed through safely.
Many items prioritize the position of Tainted Forgotten over that of Tainted Soul. For example, familiars and orbitals will follow Tainted Forgotten, thus not granting any protection to Tainted Soul.
Changes to the speed stat are only half as effective.
Area of effect damage such as Monstrance and 120 Volt apply their effect to Tainted Forgotten, making them incredibly powerful for large crowds and stationary bosses.
Care should be taken when fighting enemies spawned in Devil rooms, Shops and Black Markets, as throwing the Tainted Forgotten into a purchasable element will buy it, with potentially fatal results with deals with the devil.
Resurrecting from items such as Lazarus' Rags and Ankh should be avoided as they will cause the player to instantly die upon resurrection. However, Guppy's Collar does not cause this to happen.
Some pills don't work because they apply their effects to Tainted Forgotten. Paralysis is one example.
Throwing Tainted Forgotten into a door will cause the player to enter that room. This can be done to avoid taking damage when exiting Curse Rooms.
It can also be done to move more quickly through cleared rooms
When placing a bomb, the bomb will always appear at the current position of Tainted Forgotten, not Tainted Soul.
Be careful of Mom's Hands, because they can grab Tainted Forgotten. This will deal no damage, but will still cause teleportation back to the starting room.
The Ipecac shot when using the Horf! pill will shoot out of Tainted Forgotten, not Tainted Soul.
Using a Cracked Key or Red Key will open the door closest to Tainted Forgotten and not Tainted Soul.
Tainted Forgotten can be thrown out of the bounds of a Crawl Space which will send the player to the I AM ERROR room,
Birthright: Gives Tainted Soul the Recall item to retrieve Tainted Forgotten immediately.
Birthright + Car Battery: Recalling the Forgotten causes it to begin slowly moving towards the Soul for 5 seconds, before rapidly speeding up back to normal speed. The Forgotten can attack freely while moving. The Soul is invincible while the Forgotten is floating. As the cooldown of Recall is 3 seconds, it can be spammed to provide infinite invincibility as long as the Forgotten is not reclaimed by the Soul, almost guaranteeing the victory.
Black Powder: The powder trail is drawn by Tainted Forgotten, rendering the item difficult to use.
Brimstone: Replaces the bone throw with a short-range brimstone orb.
Dr. Fetus: Charging an attack will spawn a bomb underneath Tainted Forgotten. Using a regular attack after that will result in a usual Forgotten interaction, sending said bomb in a chosen direction.
E Coli: Offers a semi-frequent instant kill on weaker enemies Tainted Forgotten is thrown into.
Epic Fetus: After being thrown, Tainted Forgotten can prep a missile to be thrown in an arc to a crosshair controlled with the tear buttons. Unlike how Epic Fetus normally functions, the missile will not be released until the fire buttons are released. Doesn't affect the melee club attack at all.
False PHD: Good damage source in exchange for shot speed, since this stat is useless for Tainted Forgotten.
Genesis: Effectively removes all your items, touching the generated items as Tainted Soul does not grant them to the soul (though it still grants them to the Forgotten).
Gnawed Leaf: Does not deactivate when the Tainted Soul moves, regardless of whether or not the Tainted Forgotten is held. This makes it an extremely powerful item in general, and even more so when combined with a damage source that doesn't require the pressing of a fire button.
This is likely unintentional. A probable cause is that Gnawed Leaf, like many other items, is looking at the hitbox of the Tainted Forgotten instead of the Tainted Soul.
Godhead: Stat increases are applied, thrown attacks gain homing, but both the melee and thrown attacks do not have the damaging aura.
How to Jump: Will drastically increase Tainted Forgotten's launch distance when used while throwing it.
Ipecac: Tainted Forgotten's bone club will cause explosion when hitting enemies. When he is thrown the explosion may also cause the body to ricochet backwards, making retrieval much easier.
Isaac's Heart: Causes the heart to follow Tainted Forgotten like other familiars; however, Tainted Soul does not become immune to damage like other characters and both he and the heart can take damage simultaneously.
Jupiter: Acts solely as a speed boost with no fart effects.
Kamikaze!: The explosion happens on Tainted Forgotten, not Tainted Soul, allowing unlimited use with no downsides as long as you keep your distance.
King Baby: Follows Tainted Forgotten instead of floating around the room. Shooting familiars still fire towards enemies automatically.
Lead Pencil: Will fire the Lead pencil shot on the 15th swing of the bone club. Holding down the fire button will cause Tainted Forgotten to both swing and charge the bone club. Swinging, then charging and firing, counts as two swings.
Lemegeton Despite the wisps surround the 'invulnerable' Tainted Forgotten, they still take damage unless the Tainted Soul is invulnerable.
The Soul Bullets will curve away from Tainted Forgotten but not Tainted Soul.
Tooth and Nail: Contact damage applies only to Tainted Forgotten, but invincibility is applied to Tainted Soul.
The Ludovico Technique: Spawns a floating tear that slowly and loosely floats towards the Forgotten. If struck with a melee attack, it will grow in size and be launched outwards, dealing damage if it strikes something, before moving back towards the Forgotten.
Mars: Tainted forgotten will be the one dashing: can be used to control him from a distance, allowing the bones to reposition and attack every so often without putting the Soul in danger.
Mercurius: very good for Tainted Forgotten, as it allows you to go into a room and immediately throw Tainted Forgotten into another door, skipping every room until the boss/item room, whence you kill every enemy.
Midas' Touch: very good, as the petrifying effect can be activated without cost and it can do up to 69.9 damage, or 609.9 with Deep Pockets.
Mr. ME!: Mr. ME! will not follow the crosshair shown, instead it will follow Tainted Forgotten. This makes it much harder to direct where Mr. ME! is supposed to go.
My Reflection: All projectiles that would be affected by this item will return to where Tainted Forgotten is.
Panic Button: Active item is used whenever Tainted Forgotten touches an enemy, regardless of damage taken.
Pluto: direct downgrade on Tainted Forgotten, as the large size down only affects the skeleton and not Tainted Soul. This makes its hitbox smaller and also harder to pick up.
In addition to the smaller hitbox, if Tainted Forgotten is caught in a corner, Tainted Soul is unable to pick him up unless he has Recall or until he leaves the room.
Rocket in a Jar: The bomb will be a rocket only if placed while Tainted Forgotten is being thrown in the air, and will follow the same direction. As a result, if there is no ennemy between the rocket and Tainted Forgotten, the rocket will explode when touching him. Tainted Soul's movements have no impact.
Stitches: Will only teleport Tainted Forgotten, which can be used to more accurately reposition Tainted Forgotten without putting Tainted Soul in a difficult position.
Strange Attractor: Will cause enemies to be pulled towards Tainted Forgotten. This is very useful, since it makes dealing damage to enemies easier and protects Tainted Soul, pulling them away and blocking more of their shots. This is especially effective during the Beast fight.
Tech X: If thrown, Tainted Forgotten's club is surrounded by a ring-shaped laser.
Thunder Thighs: Tainted Forgotten can crush blocks by being thrown at them or being carried over them but Tainted Soul can't crush blocks. Also the speed down effect only applies when Tainted Soul carries Tainted Forgotten.
Uranus: quite good for Tainted Forgotten, as the impact from being thrown will often send the just frozen enemies sliding off to hit a wall or more enemies. This can lead to very fast room clears if enemies are clustered together.
Ventricle Razor: Portals created can be used both while Tainted Soul is holding Tainted Forgotten, which will warp them both, and when Tainted Forgotten is thrown in the portal by itself, which will warp only the bone pile and can be very effective for repositioning, especially during boss fights.
Venus: Perpetually charms enemies close to Tainted Forgotten, which synergizes well with the fact that charmed enemies can home in on it, keeping them charmed and in melee range (and away from Tainted Soul) until killed or distracted by another target.
White Pony: Can be used only when Tainted Soul is holding Tainted Forgotten or when Tainted Forgotten is just thrown. Doing so will make a trail of holy light beams behind Tainted Forgotten as it moves instead of executing the usual charge attack. This makes it possible to deal much more damage with one charge, as the player can move the Tainted Forgotten on top of an enemy to make every light beam land on them.
By loading the console command "restart 40", it is possible to play as Tainted Soul without Tainted Forgotten; however, you cannot fire, and without the presence of Tainted Forgotten, the game is only possible to beat this way because of RNG, and Tainted Soul will have the health of The Lost and the game crashes if you die.
Tainted Soul bears resemblance to a scrapped character from Antibirth called Edith. The mechanic used by Tainted Forgotten also resembles one planned mechanic Edith was going to have.
In fact, in post-launch interviews, one of the lead developers of Repentance, Kilburn, has confirmed that Tainted Forgotten is what the original concept of Edith eventually turned into during the development of Repentance. This was further confirmed by Edmund McMillen.
Certain transformations such as Guppy revert the sprite of Tainted Forgotten to appear to be standing upright.
Tainted Soul can go through the entrance to the Mausoleum without losing any health while not holding the Forgotten.
The achievement for unlocking Tainted Forgotten is "The Fettered", a possible title for the character.
"Fettered" means "bound" or "shackled", which may be linked to how Tainted Forgotten cannot move on his own and only through Tainted Soul.
Tainted Forgotten's transition screen portrait.
Tainted Forgotten does not get their active item charged when a Battery Bum pays out, unless the Battery Bum is paid by throwing the bones at him. They can still pick up dropped batteries, but the direct charges from the bum are not applied.
When fighting against Mom's Heart / It Lives, if Tainted Forgotten is not picked up before the appearance of the entrances to Cathedral and Sheol, the player will automatically progress to whichever entrance Tainted Forgotten rests on (potentially ruining an entire run).
If the Clicker active item is used to transform from any tainted character to Tainted Forgotten, only the bones will spawn (without Tainted Soul), making it impossible to move and effectively ending your run.
If the Bones part of the Tainted Forgotten is thrown into the minecart which leads to Knife Piece 2, the scene of the minecart going through the door will play out, but will not transition into the next room, softlocking the game. Restarting the game with Alt + F4 and continuing the run will fix this.
Activating the D12 can reroll the bone pile body of the Tainted Forgotten and make it disappear, leaving the bone club behind. If this happens, pressing the attack buttons has no effect. The club can still be picked up and thrown, but it only has the base throwing damage. Leaving the room returns the bone pile body to normal.
Using Genesis and picking up items as The Soul will not count to the Tainted Forgotten items.
Respawning as another character (using Lazarus' Rags, Ankh, or the Missing Poster) as Tainted Forgotten will make the character die immediately on respawn.
If picking up A Pound of Flesh, Tainted Forgotten is unable to pick shop consumables surrounded by spikes.
Using "Soul of Jacob & Esau" as Tainted Forgotten will crash the game.
Using Genesis while the Tainted Forgotten is holding Birthright will move Recall into the active item slot, preventing Tainted Forgotten from using any active items; however, using Butter! will remove Recall and prevent him from obtaining any active items for the rest of the run.