Delirium

Delirium is a boss added in The Binding of Isaac: Afterbirth †. It is the main boss of The Void 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 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 Ipecac 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 its current health percentage.
- When transforming into a segmented boss, only the first two segments share Delirium's hp; the other segments have separately generated hp and can be defeated separately.
- In rare occasions, it can transform into two or even three bosses at once.
- In rare occasions, it spawns a separate copy of the boss it transformed into.
- 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, Bumbino, and The Pile, but only Baby Plum has a unique sprite. Cannot transform into any other Repentance bosses.
- Cannot transform into Mega Satan, rendering its unique sprite unused.
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 a reddish hue to signify this change.
- Destroying non-Delirium segments when he transforms into segmented bosses can also temporarily trigger this effect.
- If Delirium is being dealt damage at a significantly high rate from multiple tear sources, he will attack, teleport, and transform at extreme speeds.

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.

Gallery

Delirium in-game.
Delirium as Monstro.
Delirium as Gemini.
Delirium as Steven.
Delirium as Dingle/ Dangle.
Delirium as Gurglings.
Delirium as Larry Jr.
Delirium as Turdlings.
Delirium as The Duke of Flies.
Delirium as Little Horn.
Delirium as Baby Plum.
Delirium as Widow.
Delirium as Blighted Ovum.
Delirium as The Haunt.
Delirium as Pin.
Delirium as Rag Man.
Delirium as Famine.
Delirium as Fistula.
Delirium as Chub.
Delirium as C.H.A.D.
Delirium as Gurdy.
Delirium as Mega Fatty.
Delirium as Mega Maw.
Delirium as The Stain.
Delirium as Gurdy Jr.
Delirium as Peep.
Delirium as Big Horn.
Delirium as Rag Mega.
Delirium as The Husk.
Delirium as The Hollow.
Delirium as Carrion Queen.
Delirium as Dark One.
Delirium as Polycephalus.
Delirium as The Wretched.
Delirium as The Frail.
Delirium as The Forsaken.
Delirium as Pestilence.
Delirium as Monstro II.
Delirium as Gish.
Delirium as The Cage.
Delirium as The Gate.
Delirium as Loki.
Delirium as Brownie.
Delirium as Sisters Vis.
Delirium as The Adversary.
Delirium as The Bloat.
Delirium as Mask of Infamy.
Delirium as War.
Delirium as Blastocyst.
Delirium as Mama Gurdy.
Delirium as Scolex.
Delirium as Mr. Fred.
Delirium as The Matriarch.
Delirium as Lokii.
Delirium as Daddy Long Legs.
Delirium as Triachnid.
Delirium as Teratoma.
Delirium as Death.
Delirium as Conquest.
Delirium as The Fallen.
Delirium as Headless Horseman.
Delirium as Krampus.
Delirium as Uriel.
Delirium as Gabriel.
Delirium as Mom.
Delirium as Mom's Heart.
Delirium as It Lives.
Delirium as Satan.
Delirium as Isaac.
Delirium as ???.
Delirium as The Lamb.
Delirium as Mega Satan.
Delirium as Hush's first form.
Delirium as Hush's true form.
Delirium's main form on Isaac's last will.
A frame of Delirium's death animation, showing a faded sprite of Isaac.

Unlockable Achievements

Delirious - Defeat Delirium.

Lil Delirium - Defeat Delirium for the 1st time.

D infinity - Defeat Delirium as Isaac.

Eucharist - Defeat Delirium as Magdalene.

Silver Dollar - Defeat Delirium as Cain.

Shade - Defeat Delirium as Judas.

King Baby - Defeat Delirium as ???.

Dull Razor - Defeat Delirium as Eve.

Bloody Crown - Defeat Delirium as Samson.

Dark Prince's Crown - Defeat Delirium as Azazel.

Compound Fracture - Defeat Delirium as Lazarus.

Eden's Soul - Defeat Delirium as Eden.

Holy Card - Defeat Delirium as The Lost.

Godhead - Earn all 5 / 9 / 10 / 12 Completion Marks on Hard mode as The Lost.

Euthanasia - Defeat Delirium as Lilith.

Crooked Penny - Defeat Delirium as Keeper.

Void - Defeat Delirium as Apollyon.

Book of the Dead - Defeat Delirium as The Forgotten.

Star of Bethlehem - Defeat Delirium as Bethany.

Suplex! - Defeat Delirium as Jacob and Esau.

Spindown Dice - Defeat Delirium as Tainted Isaac.

Hypercoagulation - Defeat Delirium as Tainted Magdalene.

Bag of Crafting - Defeat Delirium as Tainted Cain.

Dark Arts - Defeat Delirium as Tainted Judas.

IBS - Defeat Delirium as Tainted ???.

Sumptorium - Defeat Delirium as Tainted Eve.

Berserk! - Defeat Delirium as Tainted Samson.

Hemoptysis - Defeat Delirium as Tainted Azazel.

Flip - Defeat Delirium as Tainted Lazarus.

Corrupted Data - Defeat Delirium as Tainted Eden.

Ghost Bombs - Defeat Delirium as Tainted Lost.

Gello - Defeat Delirium as Tainted Lilith.

Keeper's Kin - Defeat Delirium as Tainted Keeper.

Abyss - Defeat Delirium as Tainted Apollyon.

Decap Attack - Defeat Delirium as Tainted Forgotten.

Lemegeton - Defeat Delirium as Tainted Bethany.

Anima Sola - Defeat Delirium as Tainted Jacob.

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 its main form.

Notes

Delirium is immune to Chaos Card.
Upon defeating Delirium, Isaac gains 2 charges for his active item.
Plan C can be used to kill Delirium. After Plan C is used against Delirium, when pausing in the instant of each flash from Delirium's death animation, the animation can be skipped before the death from Plan C's 3 seconds delay; this works because each flash from Delirium's death animation is still played when the game is paused.^[1] It is also recommended to position Isaac on the chest spawn location before Plan C's death occurs.
- It works with Lazarus' Rags, as it revives Isaac on the spot. However, the Completion Mark will go towards Lazarus, not the character originally selected.
- Soul of Lazarus and Inner Child revive Isaac in the same room he died in, and the Completion Mark goes toward the selected/current character.
- While playing as Tainted Lost, Birthright revives him inside Delirium's room without transforming himself into another character.
Most entities that are considered to be bosses by the game (usually identified by ones that add the boss health bar to the screen, though there are exceptions) are marked as bosses that Delirium can transform into.
Delirium's transformations have a huge amount of quirks and odd mechanics, and as such are practically impossible to fully document. A good amount of these quirks are documented below.
- Although Grubs can spawn in Larry Jr. boss fights, they are still not considered as bosses (and do not have the boss health bar), so Delirium cannot transform into a Grub.
- Delirium can transform into either form of Satan.
- When transformed into the Gurglings or the Turdlings, it only appears as one instead of two.
- When transformed into The Headless Horseman, it transforms into either the head or the body, but it doesn't transform into both at the same time.
- If Delirium transforms into Isaac and Delirium's health is low enough, it will attempt to perform Isaac's Crack the Sky attack. However, because Delirium's boss arena is a quadruple-sized room, the attack will cover half of the arena, making it extremely deadly.
- Each size of Fistula and Teratoma count as a separate boss, which Delirium can transform into just like any other boss.
- This also results in many bugs due to unaccounted for transformations. These can be viewed in the bugs section of this page.
- When transformed into two instances of the same boss (like Lokii or Sisters Vis), one of them can be killed but the other cannot (until Delirium's hp fully runs out).
  - If Delirium transforms into Pink Champion Monstro, it only takes the form of one and summons an extra Pink Champion Monstro.
    - This is also true for Pink Champion Polycephalus and Pink Champion The Cage.
  - If Delirium transforms into The Lamb's second phase then undergoes another transformation, the body doesn't disappear and can be killed, just like the original Lamb's body.
    - Killing this body will cause the screen to become darker for a period of time, just like when you kill the original Lamb's body.
    - Delirium can also transform into the Lamb's body on its own. If this happens, the body disappears when Delirium transforms again.
- When transformed into a boss that has multiple segments, the extra segments can be killed. The head, however, cannot.
  - This also applies to Gemini — Suture can be killed but Contusion cannot.
    - If Delirium transforms into Gemini, then undergoes another transformation, Suture doesn't disappear. Instead, he continues to chase Isaac as if Contusion was killed in the original fight.
- Delirium's Mom form only stomps and does not spawn enemies.
If a segmented worm-boss such as Larry Jr. or Scolex is removed via Eraser, Delirium will disappear if it attempts to turn into that boss. This will spawn the ending chest and count as having defeated Delirium. Other multipart bosses' erased pieces may be unable to spawn, but do not kill Delirium entirely.
If Delirium transforms into Mega Satan, the boss maintains the ability to spawn the Harbingers, the Super Sins, and the Fallen Angels, and can do so in quick succession depending on Delirium's health. This can make the fight much longer, as attacking the bosses spawned by Delirium will have no effect on Delirium’s actual health.
Each time Delirium transforms, its Status Effect timer resets, allowing for useful effects to be active for a large portion of the fight if re-inflicted frequently enough.
- This quirk is especially useful in conjunction with the petrification from Mom's Contacts, as it can hold Delirium in place each time it is petrified, making it a much easier target to hit.

Strategies

Just like with Hush, effects that cause long-lasting mass slowdown such as Stop Watch or the I'm Drowsy pill are very valuable, aiding the player's ability to dodge the complex tear patterns and deal with the massive spawns of enemies, which can happen in quick succession.
Delirium's main form lacks any damage scaling or armor, allowing the opportunity to deal high damage attacks against its massive base HP. Be wary that dealing significant bursts of damage against Delirium will also quickly increase its aggressiveness.
One may wish to save this fight with Tainted Eden for the last mark of a file, as Corrupted Data can make secret rooms worse.

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.^[2]
- 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.
- After Delirium finishes his death animation, the arena will change into the theme of Chest.
Delirium is one of only two enemies immune to Chaos Card, 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 The Void on any run.
- We Need To Go Deeper! can be used to access Dark Room, then create an Crawl Space there. Entering the Crawl Space and leaving it through an unintended direction will lead Isaac to an I AM ERROR room, allowing access to The Void, 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.
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 #5, 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 only bosses Delirium can't normally transform are Ultra Greed, Ultra Greedier, and any other Repentance boss except for most of those that can appear on the "main" route: Baby Plum, Bumbino, and The Pile.
- Baby Plum has a Delirium form, but Bumbino and The Pile are missing one and will simply look normal.
- Technically speaking, Delirium can transform into Ultra Greed and Ultra Greedier, but they lack unique sprites. This will never happen in a normal playthrough, as the bosses exist in different game modes.
- Delirium's Hush form can be summoned with Delirious, but it has to be used many times in quick succession in the 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. This is despite the fact that Skinless Hush is simply an incomplete red herring that does not have any AI by itself and cannot be encountered without manually spawning it in.
- Delirium can transform into a dummy spawned by the spawn dummy 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 Lil Delirium, Delirious, and D infinity.
Delirium's unlocks for Tainted characters mostly reflect the special ability of that character. For example, beating Delirium as Tainted Azazel unlocks Hemoptysis, which gives Isaac the sneeze ability of Tainted Azazel.
- The only exceptions to this are Tainted Isaac, who instead unlocks Spindown Dice, Tainted Lost, who instead unlocks Ghost Bombs, and Tainted Keeper, who instead unlocks Keeper's Kin.
- The item that most mimics Tainted Isaac's starting ability, Glitched Crown, is instead unlocked by defeating The Beast.
- The item that most mimics Tainted Lost's starting ability, Sacred Orb, is instead unlocked by defeating The Beast.
- The item that most mimics a Tainted Keeper property is Keeper's Bargain, unlocked by Satan, The Lamb, Isaac and ???'s completion marks together.
Prior to patch V1.7.8, Delirium could transform into Mega Satan.

Bugs

Bug!	If Delirium transforms into an enemy like Chub with multiple hitboxes, the game may crash if one of the secondary hitboxes is hit by Chaos Card.

Bug!	Delirium can transform into Fallen Uriel, Fallen Gabriel, Bumbino, and The Pile, but they do not have unique Delirium skins.

Bug!	If Delirium transforms into Mega Satan, the sprites will bug out. It can attack normally, but the sprite may not be visible/half-visible/only showing the hands.

Bug!	On Isaac's last will, Delirium will be shown in its current form rather than its main one. Sometimes it will show his main form, even if Isaac is killed by a different form.

Bug!	Delirium can sometimes deal unavoidable damage. For example, when transformed into Mom, the Boss will constantly target Isaac until it stomps down. If Delirium changes during this targeting phase, it can teleport into Isaac, instantly damaging him.

Bug!	Sometimes, when Delirium transforms into a segmented boss, its health will appear to be lower than it is. This is usually purely visual and returns to normal whenever it transforms again.

Bug!	If Delirium transforms into Blue Champion Larry Jr. it will, due to how Blue Champion Larry Jr.'s HP works, effectively lose a large portion of HP as its total HP will get divided into however many segments it transforms into. It will not receive the lost HP back after transforming again.

Bug!	If Delirious is used to spawn Hush, Delirium will be able to transform into Hush.

Bug!	If Delirium transforms into Gemini, Suture will stay in its enraged form separated from Contusion when Delirium transforms again.

Bug!	If Delirium transforms into Blighted Ovum, Suture will keep its usual sprite instead of using its special sprite for Delirium.

Bug!	If Delirium transforms into the Mask Of Infamy, it will only transform into the heart.

Bug!	If Delirium transforms into The Frail, it will always transform into the first phase, regardless of how much hp it has left (unless it's the Black Champion version).

Bug!	If Delirium transforms into Lokii, it will only transform into one Lokii half.

Bug!	If Delirium transforms into War's second phase, the body will be invisible (except for when it does its sobbing animation) This also applies to Blighted Ovum's Contusion.

Bug!	As of patch V1.7.8, if Delirium transforms into a champion boss, it will retain its normal appearance and color, but its attacks (and size in the case of some forms) will be those of the champion.