Missing Poster

Missing Poster is an unlockable trinket.

Effects[]

If Isaac dies to a Sacrifice Room's spikes, a puzzle piece will appear on his last will.
Dying in a Sacrifice Room while holding the trinket consumes it, revives Isaac as The Lost and unlocks the character. (Rep+) Internal Item Descriptions now hint at it with a phrase "A sacrificial offering". The text is displayed even after unlocking the character.
- If The Lost is already unlocked, the trinket works outside Sacrifice Rooms as well, reviving Isaac as The Lost.
- Tainted Lost will remain as Tainted Lost.
(Rep+) Now has a 33% chance to reveal the Super Secret Rooms upon entering a new chapter.

None of the effects will occur during any Challenges.
Resurrection items activate in a set order. Missing Poster has the lowest priority of all / the second lowest priority of all, with only Tainted Lost's Birthright effect being lower.
- If another resurrection item activates before Missing Poster while in the sacrifice room, then the player will not become The Lost, but The Lost will still be unlocked. Missing Poster will also not get used up by this.
The Lost's intrinsic Holy Mantle is not gained when revived into this way.
- If unlocked, The Lost will have his intrinsic Holy Mantle when revived into this way.
Dying to Sacrifice Room spikes specifically is not required to unlock The Lost and any death counts.
- If Isaac teleports out of the room before dying, i.e. is teleported by the spikes to Dark Room, The Lost will not be unlocked.
- Doesn't unlock The Lost in challenges.

Gulp! / Marbles / Smelter: Missing Poster functions correctly after being consumed, but still disappears after activating.

Birthright: The Lost's Birthright effect will apply after respawning as him, even if Birthright was obtained prior to dying.
Jacob and Esau: As long as one holds the poster, it doesn't matter which one dies, you'll be revived. The Lost will get both character's items.

The puzzle piece will belong to one of four jigsaw puzzles, differentiated by a grey symbol in the corner of each 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, Magdalene, Judas, or Azazel respectively.
- Each picture on the last will shows in which way Isaac has to die when playing as the respective character to unlock The Lost.
There are many puzzle pieces and more details to be found; see here.

Note that this footage only applies to the Rebirth version of this trinket and does not show the changes made to it in Afterbirth.

The Lost - It's complicated
Die in a Sacrifice Room while holding Missing Poster.

The first scenes in Ending 15 and Ending 17 show 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 a collaborative effort of the community. As such, Edmund and Tyrone had openly expressed disappointment towards the datamining.^[1]
The unlock pop-up of Missing Poster calls it "Lost Poster". It is unknown if this is a mistake or a hint.
A chest always containing the Missing Poster will appear as a room clear reward from the Dogma room, although it is impossible to access it without the use of console commands.