I always pictured Tharizdun as one of the embodiments of Cthulhu-like Elder Evils or the Far Realms. A figure bad enough to be entirely omitted from common knowledge, bad enough that any mention of efforts to revive him sends a panicked response from those who know. Like many such figures, he would seem to work best as a threat around the corner, rather than an active player.

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The Return to the Temple of Elemental Evil uses Tharizdun in this way, and might have some great inspiration for you.