The Devil Room has a chance to appear after defeating a boss in the floors between and including Basement II - Womb II. It usually contains a different set of items each time in which Isaac can do a 'deal with the devil', by trading health containers for collectable items. By taking a devil deal, this also makes it impossible to find Angel Rooms later in the run.
The chance for a Devil Room door appearing on a floor can be manipulated by meeting a set of criteria. Your percentage chance of seeing the devil room goes down if you take red heart damage on the floor, and can increase by having certain items. By default your lowest possible chance is 1%, then additionally the following conditions can lower/increase that chance:
|Condition||Rebirth Percentage (%)||Afterbirth Percentage (%)|
|Not taking red heart damage on the entire floor||+99%||+99%|
|Not taking red heart damage on the boss fight||+35%||+35%|
|Holding the Book of Belial||+2500%||+12.5%|
|Holding Goat Head||Guaranteed||+6660%|
|Holding Book of Revelations||+25%||+17.5%|
|Killing a regular coin beggar||+35%||+35%|
|Killing a dead shopkeeper on the floor||+10%||+10%|
|Holding a second Pentagram||+10%||+5%|
|Holding Black Candle||+30%||+15%|
|Saw a Devil/Angel room on the previous floor?||Percentage * 0.25 (75% reduction)||Percentage * 0.25 (75% reduction)|
|Saw a Devil/Angel room 2 floors ago?||Percentage * 0.5 (50% reduction)||Percentage * 0.5 (50% reduction)|
Combining the above conditions will give you your final devil/angel door percentage chance. By default, after meeting the conditions for an angel room to appear, you have a 50/50 chance of the door that spawns being either a devil or angel room. If you have not yet met the angel room conditions or have locked in your devil deals by taking one, it will always be a devil room door.