In Saving Yenniku, you must use the [Soul Gem] to capture Yenniku's soul and return it to his people. This is the final quest in a long chain, all available in Grom'gol, the first being Hunt for Yenniku which can be obtained from Nimboya at the north exit to the camp.
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.
is in the south-western corner of the ruins, almost completely alone. Simply use the Soul Gem while targeting him (you don't have to weaken him first or anything), and he will become friendly. Right click him to capture his soul in the gem and create the [Filled Soul Gem].