Delirium

Delirium is a boss added in The Binding of Isaac: Afterbirth †. It is the main boss of and the final boss of Ending 20.

Behavior
Delirium's main form is stationary but teleports erratically around the room and performs the following attacks:
 * Continuously sends out various overlapping patterns of white projectiles for the entire duration of the fight:
 * 20 projectiles that slowly rotate clockwise or counterclockwise slowly while expanding.
 * 16 projectiles outwards in all directions.
 * 14 projectiles that rotate clockwise, counterclockwise, or back and forth while expanding.
 * 10 projectiles that fluctuate in and out while expanding.
 * 8 slow-moving green-tinted projectiles in all directions.
 * 6 projectiles that travel outwards in a zig-zag pattern.
 * 4 projectiles outwards in a lop-sided cross.
 * 4 lines of 3 projectiles that spiral outwards.
 * A spread of 5 projectiles in Isaac's direction.
 * A burst of projectiles that travel outwards in random directions.
 * A stray short-range projectile in a random direction.
 * The projectile size per each attack is random and varies from very small to moderately large.
 * Transforms into bosses previously encountered during the run, which can include major bosses such as Mom or It Lives.
 * When it transforms into a boss, Delirium retains its own health, not the health of the boss it transformed into.
 * When transforming into certain multi-phase bosses such as Isaac, Delirium copies the phase that that boss would be at, given their current health percentage.
 * It can rarely transform into two bosses at once.
 * Whether it turns into a Champion variant or a regular variant is random and does not depend on which variant was previously defeated.
 * It is unable to properly transform into Hush's true form. In order for this to happen, very few bosses must be encountered in the run. Additionally, many of Hush's attacks do not work properly and the sprite is also unfinished.
 * Can transform into Baby Plum, but cannot properly transform into other Repentance bosses.
 * While Delirium technically can transform into the majority of bosses added, only Baby Plum has a unique sprite.
 * Occasionally, it returns to its main form to perform projectile attacks at a faster pace.
 * As it takes damage it becomes more aggressive: moving, teleporting, transforming, and attacking at a faster rate.
 * It also takes on reddish hue to signify this change.

Upon defeat, the screen starts to glitch, fills with static, and the room switches between different environments, while short scenes from the current run are played ending with Delirium screaming and bursting into a small puddle of blood.

Damage Scaling
Damage scaling is an unclear mechanic introduced by Nicalis in The Binding of Isaac: Afterbirth to provide a more challenging experience to the players, enabling some enemies to retaliate against high damage attacks by deploying an "armor" of a sort to greatly reduce the amount of damage taken. Further details and an in-depth explanation may be found here.

Delirium only has damage scaling while transformed, not while in their main form.

Trivia

 * Delirium is Isaac's manifestation of the "white light at the end of the tunnel" where the entire history of his life flashes before his eyes, moments before Isaac's death.
 * This is reflected by Delirium's death animation and its ability to shapeshift into previously encountered bosses.
 * "Delirium" is the state of mental confusion of a person who is delirious. Symptoms include restlessness, illusions and incoherence.
 * The arena can randomly change themes during the fight and is purely visual.
 * Delirium is one of only two enemies immune to, the other being The Beast.
 * Killing Delirium in a daily challenge counts as winning it. However, this does not reward any unique bonus points.
 * Unlike other completion marks, defeating Delirium does not add a mark to the post-it note. Instead, the note will become tattered (with blood on Hard mode).
 * If no other bosses have been encountered during the run, Delirium is unable to transform.
 * This is normally impossible as Mom must always be defeated in order to even have access to on any run.
 * can be used to access, then can be used to enter an I AM ERROR room allowing access to  all without fighting any bosses.
 * There was a glitch in the original version of The Binding of Isaac: Afterbirth † which caused Delirium's main form to lack an idle animation. This was soon patched.
 * In the game files, there is an unused Delirium transformation of Ultra Greed with a beard.
 * Delirium can transform into Ultra Greed if he is encountered, but this is normally impossible as the bosses never appear in the same game mode.
 * Every time Delirium transforms into another boss, it adds onto the encounters counter in the Bestiary.
 * If Delirium transforms into Mom as soon as the battle starts, it stomps in the center of the room regardless of Isaac's position, similar to how the original Mom always starts her battle by stomping in the center of the room.
 * Similarly, If Delirium transforms into Mom's Heart/It Lives as soon as the battle starts, it is accompanied by two Eyes in the same position as they are when they accompany the original Mom's Heart/It Lives at the start of its battle.
 * Prior to Booster Pack, Delirium could turn into a Lil' Haunt.
 * Also, if it transformed into Black Champion Brownie when at low health, It would spawn two Dank Squirts like the original boss would and die before the health bar actually fully depleted, without playing its death animation. This no longer happens because Delirium's Brownie form (normal and Black Champion) no longer transitions into the second phase (turning into a weakened Dangle or leaving two Dank Squirts, respectively).
 * Delirium uses the exact same sprite for both its Dingle and Dangle forms.
 * Delirium’s sprite for its Steven form is based on Ash from The End Is Nigh.
 * If Delirium transforms into Mega Fatty and performs the vomit attack, it attracts its White and Green projectiles towards it.
 * The beam attack summoned by Delirium as Isaac or ??? only covers around 1/3rd of the room starting from a corner.
 * The only bosses Delirium can't normally transform are Ultra Greed, Ultra Greedier, Hush (it can, but via cheats or bugs) and any other Repentance boss except for those that can appear on the "main" route: Baby Plum, Bumbino, Reap Creep, and The Pile.
 * Baby Plum has a Delirium form, but Bumbino, Reap Creep, and The Pile are missing one and will simply look like normal.
 * Delirium's Hush form can be summoned with, but it has to be used many times in quick succession in a same room, which is only possible via cheats or the Debug Console.
 * Delirium can even transform into the hidden incomplete Skinless Hush boss, which does not move and performs basic projectile attacks.
 * Delirium can transform into a dummy spawned by the  command in the Debug Console, and when they do, a functioning endgame chest spawns instantly, which ends the fight and triggers the ending regardless of Delirium's health.
 * There are a couple items themed around Delirium that play off of its ever-changing form - those being, and.
 * Delirium's unlocks for tainted characters mostly reflect the special ability of that character. For example, beating Delirium as Tainted Azazel unlocks, which gives Isaac the sneeze ability of Tainted Azazel.