Zamorak (Runescape); used chaos magic to corrupt land into a vast wilderness.
Gabriel Belmont/Dracula (Castlevania: Lords of Shadow)
Set ("Kane Chronicles")
Apophis ("Kane Chronicles")
Amos Kane ("Kane Chronicles")
Asriel Dreemurr, God of Hyperdeath ("Undertale")
Gabriel Belmont/Dracula (Castlevania: Lords of Shadow) utilizes Chaos magic via his Chaos Claws.
Raven (DC Comics) being the daughter of the demon lord Trigon, is a natural at dark and chaotic magic.
With a mere fraction of Chthon's power, the Scarlet Witch (Marvel Comics) could warp reality, cast spells over countless souls, worlds, and dimensions, alternate realities and permanently shift all possible futures of the entire Marvel Universe...
...and even tapping into the DC universe's version of it during the DC/Marvel crossover event.
Shuma Gorath (Marvel Comics)
The wild and world-destroying potential of chaotic magic and Chthon (Marvel Comics) himself.
Chthon's (Marvel Comics) true form.
Discord (My Little Pony: Friendship is Magic), the Master of Chaos.