Mestra was a sister of the Black Ajah.
She partook in the battle against the White Tower and its allies in Tel'aran'rhiod. She was killed by Egwene al'Vere with a ball of fire. If she was in her own body in Tel'aran'rhiod when she was killed, (which Darkfriends tend to do) her soul would be obliterated. It is likely, however, that she was using one of the stolen dream ter'angreal.