Dark titan

Magic is a literal gateway drug that allows dark titans such as Sargeras to bring evil influences to the world.

List of dark titans

 * Sargeras

Speculation
More Magic & Mayhem mentions that there are multiple dark titans, including Sargeras, trying to influence the world of Azeroth. These other titans may be ones that Sargeras corrupted and brought over to his will or perhaps they are just other rogue titans who have similar plans as Sargeras.