Of all the curses known to the sorcerers of elder lore, the curse of Mgnalah is the most vile. First the magus must obtain some part of the victim, a lock of hair or even a drop of spittle will serve as a link. Then a deep cave is found for Mgnalah will not enter where the light of the sun has touched the earth. Next is is drawn the seal of Mgnalah upon the earth and the link placed atop it. Then the incantations are recited in a whisper, for Mgnalah hears whenever his name is spoken and there is no need to shout.
Seeping down from between the spaces we know will come Mgnalah, searching for his prey. The old one will take root within his host and begin to grow, consuming flesh and mind both. In time the wretched remains will resemble nothing of this earth and mumble only of strange and terrible visions. In the end nothing human will remain, only a fragment of Mgnalah that will crawl away in search of new flesh to devour that it might grow into a greater abomination.
(From the Necronomicon of Abdul Alhazrad)