Challenge Rooms are just what their name suggests; they offer a challenge to the player. The idea is simple, the room offers you a chest (can be Regular or Golden) or an item, and if you take it, you have to kill three waves of monsters. The monsters you have to face depend on the floor in which you are in, but needless to say they get harder the deeper you are. There can only be a Challenge Room or a Boss Challenge Room in one floor, not both.
Challenge Rooms have some conditions the player must meet in order to access them. First off, you need to have full health at the start of the floor for the room to to even there. Second, you need to be on the second floor or deeper. Third, you need to have full health for the door to open, otherwise it won't let you in. Full health is based on both Red and Soul Hearts, meaning that this combination will still let you in:
As long as you either have all your Red Heart containers full, or compensate for the empty ones using Soul Hearts, the door will be open. Earlier versions of the game will let you bypass this health requirement if you have a flying upgrade, but this has been removed in the latest patch. The only way to bypass the requirement is by having one of the following items:
- Lord of the Pit
- The Nail
- Skeleton Key
When playing as ??? the Challenge Room door will always be open.
You can easily tell a Challenge Room apart from other rooms thanks to its special door:
It appears on the map as a sword:
Once inside you are presented with a "lure", which can be a chest or an item as I mentioned in the introduction. The choice whether you take the "lure" or not is totally up to you, and you can leave the Challenge Room without any negative impact on your character. If you do take it however, the door will slam shut and you will have to face three waves of monsters.
The monsters depend on the floor in which you are in, but they are always taken from the following combinations:
Basement or Cellar
- 4 Flies, 4 Pooters, and 4 Gapers
- Or 2 Hoppers, 2 Horfs, and 2 Clotties
Caves or Catacombs
- 5 Spities, 5 Hives, and 5 Hosts
- Or 5 Chargers, 5 Globins, and 5 Maws
The Depths or Necropolis
- 5 Babies, 5 Hoppers, and 5 Knights
- Or 3 Vis, 4 Leapers, and 5 Globins
The Womb or Utero and later
This is where Challenge Rooms start to get really tough as all of the waves will be Boss monsters.
- 3 Larry Jr. (short versions), 1 Chub and 3 Chargers, and 2 Monstros
- Or 1 Fistula, 1 Loki, and 1 Chub
The only way to leave a Challenge Room once it has been activated is by using Dad's Key or a teleportation item; you cannot bomb your way out like in regular rooms.
You should generally avoid Challenge Rooms when you are low on health unless there is a really sweet item being offered as the risk of taking damage is too great. As a final note, completing the Challenge Room can also drop "room clearance" rewards such as regular rooms so you may end up getting an extra Soul Heart or Bomb.