The Chest is a floor found after opening the end chest at the end of the Cathedral while holding the Polaroid trinket. It is a brutal bonus level featuring rooms with multiple bosses; it contains no extra rooms besides the boss room containing ??? (a.k.a. Blue Baby) as the boss (which is a reference to ending 10). The first room you start in contains 4 locked chests, each with an item inside that may potentially give you an advantage for the difficult challenges that await.
Every chest on this floor, regardless of its type, contains an item to assist you. This means if you're feeling brave, it might be a good idea to explore the floor entirely to possibly get more items. If possible, acquire Guppy's Tail and save up your keys to have a better chance of spawning golden chests here, or get the Left Hand trinket to aquire Devil Room items from the Red Chests that will spawn.
Rooms in The Chest frequently contain two to three bosses or mini-bosses. Despite their numbers, their health is much smaller than usual so eliminate the most threatening foe first before dealing with the other enemies. Occasionally, you might run into commonly found lesser enemies though in numerous amounts.
It's important to note that you cannot bomb your way out of these rooms by any means; you must defeat every enemy in each room to continue. This ties back to the ending where it is implied that Isaac locked himself in the chest, and cannot get out.
**NOTE: You must have The Polaroid every time you wish to enter the Chest. So, if you want to achieve all the 'Chest' achievements, you'll have to find the Polaroid at least 7 times. Note: Since the 1.4 patch, Mom drops The Polaroid, so you don't have to depend on luck anymore.
Bosses encountered in The Chest
- The Bloat
- Daddy Long Legs
- 2 Carrion Queens
- 2 different Harbingers
- 3 different Sins (like other rooms with normal Sins, it's a chance for each of them to be a Super Sin)
- 2 identical Super Sins (either Super Sloth, Super Wrath, or Super Lust)
- 2 Husks
- 3 Lokis
- 3 Widows
- 2 Monstro IIs
- 2 Chubs
- 2 Wretched
- 2 Gurdy Jrs.
- Gurdy (The only time she appears as a level boss as opposed to an end level boss)
Main boss at the end of level
- When using the Best Friend spacebar item against Daddy Long Legs, the player is instantly killed when the decoy explodes, regardless of their remaining health.
- If you blow up a slot machine or a beggar on the chest, an item will appear instead of it's normal drops. However, this will not work for blood banks. (Needs confirmation)