It is unknown of what became of Moytirra, but such a powerful mage that managed to challenge Mazzarin would unlikely just disappear. A bigger mystery is why her Warlock order followed Mjarin and then Mydred if she was still around. Perhaps she died, but that is subject to conjecture since such an important being would have likely been mentioned in the total codex or would have been discussed about in the journal.
Moytirra also would have been sought after by the Dark--whether to be raised as an undead or for her ability to command the Warlocks. What became of her, where she is, and where her loyalties truly lie are a mystery.
Moytirra could utilize many fire dreams, mentally debilitating dreams, teleportation spells, transforming dreams (units into small meaningless animals), Moytirra could fly over long distances, and she could use most spells that were darker versions of Mazzarin's spells.