Forest Dweller

"You're trying to destroy everything. A being... born of Cosmos, you can never hope to achieve what I did. Cosmos- what a pathetic excuse for an element. Now leave, and never return."
Comprehension returns to the eyes of Venach, and he grins.
"I'm afraid that's no longer an option."
"What?"
"I'll kill you, and overpower you once and for all!"
"IMPUDENCE!" You scream. The force blows Venach back. "I took you in, made you a soldier in that land, and this is how you repay me? You forget, -I- can damage souls. I'll kill you and torment you for eternity."
Venach seems to pause for a second, then charges, a red and white glow covering his body.
"Cosmos, Darkness and Chaos. The CDC. The Ultimate Destroyer."
"What?" Venach stops in his tracks.
"You're weak. You could never hope to pull it off."
"SHUT UP!"
"You. Boy." You gasp and answer, both mentally and telepathically.
"Yes?"
"Do you really want your body back?"
"I want Venach dead." Hearing yourself talk to yourself makes you feel slightly strange, but you continue.
"Then that's one thing we have in common. I'll give you a choice... you either destroy him on his own, and I'll find another host- or I'll destroy him and we will fight for this body."

A difficult decision. Can you hope to defeat Venach without Cassio's help?