Rule 811.1.d.3.a
A play effect of a permanent played from facedown references information from its source when determining what battlefield it was played from and what targets are legal. Example: A unit reads in part “When you play me, kill another friendly unit.” If it is played from the facedown zone, the play effect must choose a friendly unit at the battlefield the unit was played from. If the unit is no longer on the board when the play effect attempts to finalize, it will not be able to choose a legal target. If the unit is no longer on the board when the play effect attempts to resolve, the play effect will not be able to reference the information of where the unit was played from, and it will mistarget.