Missing Poster

How do you unlock Missing Poster?

In The Binding of Isaac, Missing Poster is unlocked by earning the The Lamb completion mark as Isaac (Character).

[Before Afterbirth] If Isaac dies to a Sacrifice Room's spikes, a puzzle piece will appear on his last will. / [Afterbirth] Dying in a Sacrifice Room while holding it unlocks The Lost. If The Lost is already unlocked, revives Isaac as The Lost if he dies anywhere, then disappears. [Repentance+] Also has a 33% chance to reveal the Super Secret Room when entering a new floor.