- Delirium will teleport around the room, sending out an array of white projectiles and green projectiles that explode like Ipecac.
- Its most notable (and dangerous) attack is that it will transform into bosses previously encountered during the run, including a few of the "final" bosses (Mom, It Lives, etc.).
- While transformed, it will attack with its white projectiles along with the attacks of the boss it is transformed into, though it cannot attack with its explosive projectiles.
- When it transforms into a boss, Delirium retains its own health, not the health of the boss it transformed into.
- The attacks of the transformed boss are dependent on Delirium's current HP. For example, if Delirium transforms into War while at <50% HP, it will be horseless and run at Isaac.
- It can rarely transform into two instances of the same boss.
- Whether it turns into champion variants or regular variants is random and does not depend on which variant was previously defeated. For instance, it can turn into black Gurdy, even if the regular Gurdy was the defeated variant, and vice-versa.
- It is unable to properly transform into Hush.
- Progressively, it will alternate through each form faster and become more aggressive.
Upon defeat, the screen will start to glitch and fill with static while short moments from the current run are played. After being defeated, Delirium will burst into blood and drop a Final Boss Chest, playing the Final Ending.
Delirium as Gurglings.
Delirium as Little Horn.
Delirium as Turdlings.
Delirium as Rag Man.
Delirium as Famine.
Delirium as Gemini.
Delirium as Steven.
Delirium as Blighted Ovum.
Delirium as The Duke of Flies.
Delirium as The Husk.
Delirium as Larry Jr..
Delirium as The Hollow.
Delirium as The Haunt.
Delirium as The Forsaken
Delirium as Widow.
Delirium as The Wretched.
Delirium as Pin.
Delirium as The Frail.
Delirium as Monstro.
Delirium as Monstro II.
Delirium as Gish.
Delirium as Fistula.
Delirium as Teratoma.
Delirium as Gurdy Jr..
Delirium as Rag Mega.
Delirium as Gurdy.
Delirium as Big Horn.
Delirium as Pestilence.
Delirium as Polycephalus.
Delirium as The Stain.
Delirium as Chub.
Delirium as C.H.A.D..
Delirium as Carrion Queen.
Delirium as Mega Fatty.
Delirium as The Cage.
Delirium as Dark One.
Delirium as The Adversary.
Delirium as Peep.
Delirium as The Bloat.
Delirium as Mega Maw.
Delirium as The Gate.
Delirium as Brownie.
Delirium as Mask of Infamy.
Delirium as Sisters Vis.
Delirium as War.
Delirium as Loki.
Delirium as Lokii.
Delirium as Blastocyst.
Delirium as Scolex.
Delirium as Death.
Delirium as Conquest.
Delirium as Daddy Long Legs.
Delirium as Triachnid.
Delirium as Mr. Fred.
Delirium as Mama Gurdy.
Delirium as The Matriarch.
Delirium as The Fallen.
Delirium as The 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. (First phase only)
Delirium's main Form on Isaac's last will.
- Delirium's boss music is named after Delirium itself.
- Delirium is the only enemy immune to Chaos Card.
- When it changes into an enemy like Chub with multiple hitboxes, if one of the secondary hitboxes is hit by Chaos Card, it will kill Delirium in one hit. However, this can crash the game, so it's best to simply not use the Chaos Card during the battle.
- As a single stage boss, Delirium can be instantly killed by Plan C, but Isaac will still be killed first by its effects, as Delirium's death animation is longer than 3 seconds.
- Killing Delirium in a daily run counts as winning it.
- 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 fought, Delirium will be unable to transform.
- Delirium can transform into bosses featured in custom mods, though it will not have unique sprites by default. This is probably because Delirium takes other bosses' A.I. and uses them in sync with his own sprites, if he has the appropriate boss sprites.
- Delirium can transform into mini-bosses, such as the Seven Deadly Sins, but they don't have Delirium skins. 
- Delirium can transform into either form of Satan.
- Delirium can turn into Hush's second form if it's the only boss encountered in the run or very few bosses have been encountered in the run; this can be done with the debug console,  though this is technically a bug, as the sprite is unfinished and many of the attacks don't work properly.
- It can also transform into Hush if the form has been encountered by spawning it in with Delirious.
- Every boss in the game (with the exception of the fallen angels and Ultra Greed) has Delirium versions complete with their sprites and animations, including the unfinished Delirium version of Hush.
- If it transforms into Fallen Uriel or Fallen Gabriel, it will use their Fallen skins, not their Delirium skin.
- If it transforms into Mega Satan, the Sprite will bug out. It can attack normally, but the sprite may not be visible/half visible/only showing the hands.
- The Gurglings originally lacked unique Delirium sprites, though this was later patched in a Booster Pack.
- 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.
- All parts of Fistula and Teratoma can be killed (though they have much greater health than normal), and the room will remain empty until Delirium transforms again. Even though the segments have been killed, Delirium itself has not been killed.
- When transformed into two instances of the same boss, one of them can be killed but the other cannot.
- When transformed into the Gurglings or the Turdlings, it will only appear 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.
- The sizes of Fistula and Teratoma depend entirely on chance.
- 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.
- Unlike the real Mom, Delirium's Mom form will only stomp and will not spawn enemies.
- The word "delirium" means "madness" or "derangement". A delirium is a state of mind characterized by disorganized behavior, incoherence, hallucinations, and delusions. Delirium is meant to represent Isaac's current state of mind; he is experiencing his own delirium, and the battle is meant to represent his battle against his own madness.
- Another word with a similar meaning to delirium is “dementia”.
- 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 might be reflected by Delirium's death animation and its ability to shapeshift into different bosses you previously encounter.
- Every time Delirium transforms into another boss, it adds onto the encounters counter in the Bestiary.
- If Delirium transforms into Gemini then undergoes another transformation, Suture doesn't disappear. Instead, he will come after the player like the normal version does after Contusion is killed.
- 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 real Lamb's body.
- Prior to Booster Pack #5, Delirium could transform into a Lil' Haunt.
||Blighted Ovum • Dingle • The Duke of Flies • Famine • Fistula • Gemini • Gurglings • The Haunt • Larry Jr. • Monstro • Pin • Steven • Widow |
Dangle • Little Horn • Rag Man • Turdlings
||Carrion Queen • C.H.A.D. • Chub • Dark One • Gurdy • Gurdy Jr. • The Hollow • The Husk • Mega Fatty • Mega Maw • Peep • Pestilence • Polycephalus • The Wretched |
The Forsaken • The Frail • The Stain
Big Horn • Rag Mega
||The Adversary • The Bloat • The Cage • The Gate • Gish • Loki • Mask Of Infamy • Monstro II • War • Mom |
||Blastocyst • The Bloat • Conquest • Daddy Long Legs • Death • Lokii • Mama Gurdy • Mr. Fred • Scolex • Teratoma • Triachnid • Mom's Heart • It Lives |
Sisters Vis • The Matriarch
| Blue Womb
||Satan • Isaac|
||??? • The Lamb • Mega Satan|
| Greed Mode
|| Ultra Greed |
| The Void
|Chapters 1-4||The Fallen • The Headless Horseman|
|The Binding of Isaac: Rebirth|
|Achievements||Attributes||Bosses||Cards and Runes||Challenges||Chapters|