Doppelgänger

Doppelgangers are usually magical in nature and take the form and shape of a character, sometimes copying more than just the physical form, being able to copy the memories of the original. For some doppelgangers, they can be recognized with a kiss, as, though the disguise is almost perfect, they cannot fully hide their mouths, which they rely on most to spew their deadly lies.

The Demon of Lies
During the Temple of Lies(?) session, the final boss took the form of a Doppelganger, appearing as Madella the Gunslinger, Darreck Kel'lara, and Deckerd Cain. Unfortunately for Darreck, Deckerd had not only cut off the testicle of the real Darreck, but also slit his throat. It was only after Darreck's disappearence and Deckerd's copying did the stone tablet give a clue as to the nature of identifying the demonic doppelganger.