A refugee of the Burning Legion, Nargul is a powerful Nathrezim Dreadlord. Using his pheonominal psychic powers Nargul is known to manipulate events from the darkness which he was spawned, tailoring the world to an ends known only to a select few.
When I created this character, I intentionally made him a Dreadlord who serves two functions: 1) as an all-in for my dangled idea of a Burning Legion invasion subplot, and 2) as a way to better portray the Nathrezim mind control powers which exist in Warcraft lore, but seem to be neglected in practice. In the finished campaign, though, he is far toned down to what I really had in mind-- Nargul was going to be so depraved, so vicious, his mind rape would literally have shown Kurumi something exceptionally devestating like actual rape-- but fortunately cooler heads prevailed and I went with something a little more personal for Kurumi as a character (preying on her inferiority complex).
Nargul is probably the sickest SOB I made in the campaign. I leave what he did to Kurumi up to your imagination, although I can assure you it wasn't pleasant. Recent Blizzard lore suggests it isn't so easy to kill Nathrezim, so he may come back again someday....