The Changeling

The Changeling is Tzeentch's favourite stolen face. It has shapeshifted so often even it has forgotten the original, which the Changer keeps as a leash. Mortals, daemons, and gods have all been the joke: Slaanesh's hair cut in the night, Nurglings on Khorne's throne, a Warboss handed a cannon that fired itself.

It wore a Dark Angels scout and then Sammael to talk Azrael into the Siege of the Fenris System. It wore dead men to reach Ahriman with a gift of annihilation, then had the errand cancelled mid-sentence because Tzeentch had a newer plot. The true face flashed once and was erased from the sorcerer's memory.

You do not unmask the Changeling. You wait to see which friend was never there.