Shadows of Doom: The Shadow of the Avatar, Book I by Ed Greenwood
It was the eve of the Time of Troubles. The chaos of spilled blood, lawless strife, monsters unleashed, and avatars roaming Faerûn was still to come. Unbeknownst to mortals, the gods had been summoned together, and among them was Mystra, grown proud and willful in the passing eons. With the others, she was about to be stripped of her godhood.
The secret of her power gave her an idea. She made certain preparations, looking always for one who would be her successor . . . But until that person's ascension, her power must be preserved.
A lone mortal must carry the greater share of her divine energy until the power could be reclaimed, and it was the fate of this mortal to risk being destroyed or driven wild, involuntarily and without warning.
This was the occaision of Elminster's Doom.