Bring the Filled Soul Gem to Nimboya.
I have reshaped the Mind's Eye into a Soul Gem. Its purpose is twofold: you must use it to stun Yenniku, and once stunned place it upon his head to entrap his soul.
Then, bring the gem to Nimboya.
Yenniku dwells with Zanzil the Outcast, at the Ruins of Aboraz, along the Crystal Shore on the distant Cape of Stranglethorn.