Curses are effects that are effective during a whole floor. A curse will be announced at the top of the screen upon entering a floor. Curses were added to the game in the Wrath of the Lamb expansion.

Curse of the LostEdit

Curse of the Lost increases the size of the current floor by 4 rooms and can affect any floor, including Sheol and Cathedral.

Curse of DarknessEdit

Removes the map, replacing it with a single black room with a question mark in the middle (same as the devil room) for the duration of that floor. Having the Treasure Map or the Compass will not override this curse, and neither will The World or The Sun tarot card.

• It is a good idea to draw a copy of the map and make a note of things you cannot access immediately, such as locked item rooms or chests. This can also enable you to locate the secret room.
•  If you look closely, there is also a key icon in the centre, and when travelling between rooms a heart icon will flash. This is most likely a bug.

Curse of the LabyrinthEdit

The two stages of a floor are replaced by one large map, larger than the typical size of both stages combined. The floor is usually given the suffix "XL", however this does not happen on The Depths or Necropolis [bug?]. The map features two treasure rooms and two bosses but only one shop, one secret room, etc. Bosses are found next to each other, requiring you to defeat the first boss to access the second. You cannot get Curse of the Labyrinth in Sheol, The Cathedral or The Chest , since those levels have only one stage.

• If you get the curse on Utero (Utero XL) you will have to fight against one of the bosses from Utero (ex. Lokii) and Mom's Heart/It Lives
• Note: If Cellar 1 / Basement 1 is affected by this curse, both treasure rooms will be locked and require a key.
• As of v1.2, the first boss of The Depths/Necropolis XL floor doesn't drop an item. This is most likely a bug [fixed as of  WotL1.48].
• It is possible for an XL floor to have two areas that are impossible to travel
  
  between without explosives or teleportation. As a result, the player can become trapped and unable to complete the floor. The two areas are connected by one or both secret rooms however, if Greed spawns inside the secret room only one of the four walls will be bombable, causing an impasse [probably fixed as of WotL 1.48].
• Using the Forget Me Now on a Labyrinth floor will spawn a new X-2 level, rather than a second Labyrinth level.
• Bug: If this happened in Depths/Necropolis or the Womb/Utero, the music for the first boss becomes the music for the second boss (i.e. either the music for Mom or Mom's Heart/It Lives).
• Bug: Only one boss skull will show on the map at a time.
• Using The Dice on the first boss's item will give you items normally dropped by the second boss (e.g. a Cube of Meat if the second boss is a Harbinger or a Devil Room item if the second boss is the Fallen ).

Chances for a Curse to happenEdit

The base chance for a curse to appear 1.25%. This chance is replaced depending on which secrets have unlocked:

• The Womb: 3.33%
• Everything is Terrible (kill Mom's Heart six times): 10%
• Defeated Isaac at least once: 20%

The Chance is now calculated as follows:

1. the game uses the Base Chance to see if Curse of the Labyrinth can be triggered. When not on the first level of a floor and on Cathedral/Sheol this curse cannot be triggered.
2. If Curse of Labyrinth is not triggered, New Chance = 2 * Base Chance is calculated.
3. Use New Chance to see if Curse of the Lost is triggered.
4. If Curse of the Lost is not triggered, use Chance again to see if Curse of Darkness is triggered.