Only available in the Wrath of the Lamb expansion, Boss Challenge Rooms are similar to regular Challenge Rooms but have some differences. Their reward is greater, but so is the challenge the player must face. Challenge Rooms are mutually exclusive, you cannot have both a Boss and a regular Challenge Room in the same floor.
Boss Challenge Rooms can spawn in any floor except the first but require that the player has just one Red Heart left in order to enter (Soul Hearts do not count). Just like regular Challenge Rooms, the door is always open when playing as ???.
The door is the same as the regular Challenge Room's but has a skull under the swords:
The map icon is a bloody sword:
The "lure" found in these rooms will always be an item, but the catch is that the monsters will always be bosses.
There are only two waves of bosses in Boss Challenge Rooms but in the deeper floors you will face two bosses at the same time. The following combinations are possible:
You are pitted against Monstro and then three Larry Jr.s (short version).
Here you will have to fight Chub which is then followed by Peep.
The hardest of the Boss Challenge Rooms, here you will have to fight Monstro II and Loki at the same time.
I try to always do the Boss Challenge Room if the item is worth it but don't risk it if your health is low. Remember that if you have Dad's Key or a teleport item you can grab the item and escape.