Cloak and Dagger may be 2E, but it is still an excellent product, very much worth picking up if you come across it (even though parts of it have been covered in LOD).
Anyhow, from a cursory reading of C&D:
In Tarsakh 1369, Fzoul discovers an alliance between the High Imperceptor of Mulmaster and the beholder Orox.
In Marpenoth Fzoul seizes control of the Cult of Xvim. The Cultists "first strike in their god's name" is the killing of High Inquisitor Ginali.
Warfare between the forces of Fzoul and the Cyricists of Mulmaster in Uktar of the same year results in the deaths of Orox the beholder, "more than half of Cyric's Mulmastran worshipers," and the High Imperceptor of Cyric (who dies by Fzoul's hand).
The "long-hidden" High Imperceptor of Bane is captured during the chaos and taken back to Zhentil Keep, his hands and jaw broken, to endure "months of torture....beneath the Keep at the Heart of the Hand."
As for the fate of the High Imperceptor, I'm not sure if its ever mentioned (he may be still strapped to a table in the dungeons beneath the Keep!), but he may also have been consumed during the ritual that produced the Tyrantfog; while C&D mentions "captive allies of Manshoon" and "numerous "heteritcal" Cyricists" becoming blood sacrifices, Fzoul may have thrown the High Imperceptor in as well....
As for the Tyrantfog, it was the result of the aforementioned ritual and sacrifices. It produced a "dark gray fog" that settled over the Moonsea and lands as far south as Starmantle. The fog harmed Cyricists and places of Cyric-worship, and caused "pockets of power attributed to Bane," including temples, shrines, and all baneliches caught within its' reach to explode into flame. The fog then receeded, imbuing Fzoul with its power and turning him into Xvim's Chosen.
Hope that helps,