- Delirium will teleport around the room, sending out an array of white projectiles and green projectiles that explodes 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 or three instances of the same boss.
- 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 drop a Final Boss Chest, playing the Final Ending.
- Delirium's boss music is named after Delirium itself.
- 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.
- 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 turn into a Lil' Haunt, which is the only enemy it can turn into that is not a boss.
- 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) 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.
- If it transforms into Ultra Greedier, it will use the regular Ultra Greedier skin and not a Delirium skin.
- When transformed into a boss that has multiple segments, the extra segments can be killed. The head, however, cannot.
- This also applies to Gemini - Contusion can be killed but Suture 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.
- The sizes of Fistula and Teratoma depend entirely on chance.
- In the game files, there is an unused Delirium transformation of Ultra Greed with a beard.
- 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.
Delirium's main Form on Isaac's last will.
Delirium as The Duke of Flies.
Delirium as The Husk.
Delirium as Larry Jr..
Delirium as The Hollow.
Delirium as The Haunt
Delirium as Gurdy Jr..
Delirium as Gurdy.
Delirium as Mama Gurdy.
Delirium as Blastocyst.
Delirium as Mom.
Delirium as Mom's Heart.
Delirium as It Lives.
Delirium as Monstro.
Delirium as Monstro II.
Delirium as Gish.
Delirium as Mega Fatty.
Delirium as The Cage.
Delirium as Gemini.
Delirium as Blighted Ovum.
Delirium as Steven.
Delirium as Mega Satan.
Delirium as Hush (unused and unfinished).
Delirium as Dark One.
Delirium as The Adversary.
Delirium as Mask of Infamy.
Delirium as Peep.
Delirium as Polycephalus.
Delirium as Fistula.
Delirium as Teratoma.
Delirium as The Fallen.
Delirium as Krampus.
Delirium as The Wretched.
Delirium as Chub.
Delirium as C.H.A.D..
Delirium as Loki.
Delirium as Lokii.
Delirium as Isaac.
Delirium as ???.
Delirium as Famine.
Delirium as Pestilence.
Delirium as War.
Delirium as Death.
Delirium as Conquest.
Delirium as The Headless Horseman.
Delirium as Daddy Long Legs.
Delirium as Triachnid.
Delirium as Pin.
Delirium as Scolex.
Delirium as Satan.
Delirium as The Lamb.
Delirium as Mega Maw.
Delirium as The Gate
Delirium as Sisters Vis
Delirium as Mr. Fred
|Chapter 1|| Blighted Ovum • Dingle • The Duke of Flies • Famine • Fistula • Gemini • Gurdy Jr. • Gurglings • The Haunt • Larry Jr. • Monstro • Pin • Steven • Widow |
Dangle • Little Horn • Rag Man • Turdlings
|Chapter 2|| C.H.A.D. • Chub • Dark One • Gurdy • Gurdy Jr. • The Hollow • The Husk • Mega Fatty • Mega Maw • Peep • Pestilence • Polycephalus • Carrion Queen • The Wretched |
The Forsaken • The Frail • The Stain
Big Horn • Rag Mega
|Chapter 3|| The Adversary • The Bloat • The Cage • Carrion Queen • The Gate • Gish • Loki • Mask of Infamy • Monstro II • War • The Wretched • Mom |
|Chapter 4||Blastocyst • The Bloat • Conquest • Daddy Long Legs • Death • It Lives • Lokii • Mama Gurdy • Mom's Heart • Mr. Fred • Scolex • Teratoma • Triachnid|
|Chapter 5||The Fallen • Isaac • Satan|
|Chapter 6||??? • The Lamb • Mega Satan|
|Greed Mode|| Ultra Greed |
|Chapters 1-4||The Fallen • The Headless Horseman|
|Achievements||Attributes||Bosses||Cards and Runes||Challenges||Chapters|