Tephra

Tephra is a human wizard. Formerly a heroic adventurer, she joined the mysterious Allabar after succumbing to her desire for power and opening a planar gateway to the Far Realm.

Hero
She trained herself from an early age and, seeking more power, applied to join the Starfire Academy in Silverstone. When she was rejected, she journeyed to Brightstone to look for adventures that could get her the power she so desired. Joining with Nuraya, Ro, Gryff, and Harthur, she investigated the disappearance of a powerful wizard over seven hundred years ago, eventually coming into the possession of a tome covered with faces that constantly stretched and moaned.

Tephra was transported to Estorium along with the rest of her party, and was eventually tasked with finding and stopping a mind flayer who was attempting to open a gateway to the Far Realm. They delved deep into the old temple of Boccob within the ancient city of Derenath, located the gateway, and slew the mind flayer. However, instead of closing the gateway, Tephra opened it fully, claiming the power within and allowing the nightmarish hordes of the Far Realm access to the world. She was last seen surrounded by aberrations, attempting to control them as her former party members fled.

Herald of Allabar
Tephra was later encountered by the Challengers of the Unspeakable along Duvik Pass, attempting to open the prison containing the primordial Hoshotath. She claimed to be serving a being known as Allabar, who had given her the ultimate power she had always dreamed of, and who would soon remake the world in his image. Hoshotath would destroy the world's defenders, paving the way for her master's arrival. She was killed by Kalth, but her body erupted into a mass of writhing tentacles, and her telepathic voice told the Challengers that they would see her again...

Harbinger of Chaos
Tephra's next encounter with the Challengers was after they emerged from the Tomb of Horrors with the soul gems of Acererak. She asked for gems, and when they refused she summoned foulspawn and attacked them, but fled when the battle began turning against her. She met the Challengers again, further up the path, and when they again denied her request she summoned more foulspawn and attacked once more, teleporting away again when they began to gain the upper hand. She met them a final time only a few miles outside Silverstone; this time she did not fight them personally, instead summoning the aberration Shoth-Gorag to kill them before teleporting away.