What happens if I complete the title screen puzzle?

I have beaten Isaac the Cathedral level once, and when I returned to the title screen, there was a rectangle with what appeared to be a piece of a puzzle in it. I infer that I can complete the puzzle by beating Isaac with each character. Is this the way to complete the puzzle, and what happens when I complete it?


Your guesses are correct. The puzzle is completed by beating the Cathedral six times.

Here it is after my second victory against Isaac:

unlock 2

Completing the puzzle unlocks the Polaroid trinket, an item that shields Isaac when brought to 1/2 heart, but that also

unlocks the final level, the Chest, if you beat Isaac while equipping it. This is the only way to access it and get to the TRUE final boss, ???. Beating Isaac with it also unlocks Dad's Key.