- For information on the character, see ???.
???, a.k.a. The Blue Baby, is the boss of The Chest as well as one of the several final bosses. His encounter is very similar to Isaac, except he is more powerful in that he fires more tears in retaliation.
??? will appear lying down in the center of the room, crying on the floor. He shoots 12 high-velocity tears in a radial burst, some of them being purple and slightly homing, and will retaliate with streams of tears when hit, some of which may also be homing. He will also spawn Flies continuously.
At 66% of his health, a light will shine on ???, and he jumps from the ground. He will then periodically shoot 4 large purple homing tears in all directions. He will still retaliate with tears when hit and will continue to spawn Flies.
Triggered near 33% of ???'s health. ??? will grow angel wings, smile, and start floating. White beams will be randomly shot down from the sky (similar to Crack The Sky and White Pony), Angel Babies will start spawning frequently (up to a maximum of 6), and the same homing projectiles from before will be shot in six directions at the same time. At this point ??? will no longer retaliate with tears when hit, and will spawn Flies less frequently.
Eternal ??? acts almost the same as its normal counterpart, but he teleports more randomly and frequently than Eternal Isaac. He also slowly regenerates HP during Phase 1 like Isaac.
- Using The Bible in the room where ??? is fought will instantly kill the player (similar to Isaac and Satan) unless they have The Wafer item.
- Like in the Isaac fight, using items with a high fire rate (such as Technology 2) is dangerous in this fight, as ??? will retaliate with a large amount of tears upon each hit. This also applies to effects which apply damage rapidly, such as Mom's Knife.
- Applying slow effects such as Little Gish or Spider Bite to ??? will not affect the speed of his homing tears, but will slow all normal tears.
- Activating Mom's Bra or triggering the special effect of Mom's Contacts will prevent ??? from spawning enemies or attacking at all—including when damaged—for the duration of the effect.
- ??? cannot be damaged by the Crack The Sky light beams, or by his own. However, any Angel Babies in the fight can take a small amount of damage from the light beams.
- ??? is first added to the Wrath of the Lamb DLC for the original game.
- This boss is a reference to Ending 10, where Isaac finds ??? in the giant chest.
- ??? is technically an alternative version of Isaac (as a boss), having the same attacks only with some variants.
- ??? Is also a reference to BlueBaby, Edmund's Newgrounds name.
- In the second phase, ???'s tears are incorrectly shaped on his sprite, while in the third phase, his tears are falling down correctly.