From Binding of Isaac: Rebirth Wiki
The Missing Poster is a trinket.
- If Isaac dies in a Sacrifice Room while holding it, a particular puzzle piece will be displayed as cause of his death on his last will.
- Dying in a Sacrifice Room while holding it will outright unlock The Lost, regardless of current character.
- If Isaac dies while holding this trinket and has already unlocked The Lost, he will be revived as The Lost and Missing Poster will be consumed.
- If The Lost dies while holding this trinket, he will be revived in the previous room and Missing Poster will be consumed.
- The Lost will not be unlocked if the player is in a Victory Lap or RERUN.
- None of the effects will occur during any Challenges.
- Even if Holy Mantle has been unlocked as one of The Lost's starting items, if any character dies while holding the Missing Poster, The Lost will not hold the Holy Mantle upon revival.
- If Gulp!, Smelter, or Marbles were used to absorb the poster, its effects will apply and can be used to unlock The Lost and respawn (but the effect still only applies once).
- The puzzle piece will belong to one of four jigsaw puzzles. To which one is indicated by a tiny grey symbol to the top left of the piece.
- The symbol can either be a pixel, a 2x2px square, a 3x3 px square, or four pixels arranged like a plus-sign.
- Each jigsaw puzzle displays an altered last will for the characters Isaac, Maggy, Judas, or Azazel respectively.
- The first scene in Ending 15 shows The Missing Poster pinned to a utility pole, with a silhouette of Isaac's mother who seems to be searching for Isaac in the background.
- There are many puzzle pieces in total. Assembling all of the jigsaw puzzles and figuring out what they meant was initially designed to be collaborative effort of the community.
The Missing Poster in Ending 15.