Hmmm.... You basically want it so that when this person uses his death form, no one knows who the player is keeping his other units "safe".
What I would suggest is that when your unit is in "death form" you give ownership to "player 12" of the unit, and you give unit sharing from player 12 to the person that triggered it. This would make sure that no one knows who the person in death form is.
Hope it helps!