Mordiggian is a Great Old One and is worshipped by ghouls. When he appears, all fire and heat is sucked into his swirling,void-like body, instantly lowering the temperature by many degrees, and filling the area with a deathly cold and still air. All within the presence of the Great Ghoul are blinded by the weird changing and dazzling form of the necromantic god.
Mordiggian attacks by engulfing victims, sucking away their life force, and dissolving their bodies. Nothing remains of the Charnel God's prey, and they are never seen again in the waking world or in the Dreamlands.
However, Mordiggian does not appear to be especially malevolent ("Mordiggian...was a benign deity in the eyes of the inhabitants of Zul-Bha-Sair"), and has been known to spare those who have not personally offended him or his followers (the ghouls); when a trio of Necromancers snuck into the temple, the wizards were torn apart by the Ghoul priests while Phariom and his newly revived wife were spared at Mordiggian's bidding.
As one of the Necromancers said "Mordiggian's Wrath, though rarely loosed, is more terrible than any other deity. And it should not be thought by wise men to break into his sacred house." So while Mordiggian is attributed with immense powers of destruction, he is apparently a calm and benign deity.