Shaman Kortik fears New Tuchanka will descend into a clan war unless two rival warriors perform the Rite of Union. A krogan war will likely be bad news for the Nexus and the stability of Heleus.
Speak to shaman Kortik in New Tuchanka nearby the bar.
Enter a wild fiend habitat (navpoint) and place the lure. A couple of wraiths attacks, and then comes the Fiend. Defeat it - in this case that means that the fiend has been tamed.
Report to Shaman Kortik. Of the two rivaling krogans Varka is up for the challenge, but Gren refuses to fight together with her. So you get task to talk to Gren about it. Make him agree to take part in the Rite of Union. Bringing Drack helps - he knows what to say. Gren suspects that Varka will shoot him as soon as the fiend is dead. So he agree to do it the condition that Ryder is there covering his back.
Choose "Go to Arena" to begin the Begin the Rite of Union. There waitss an unpleasant surpise. The fiend Ryder lured is a female, and two males followed her. Both Varka and Gren liked that - now it will be a real fight. So, better start defeating them.
When it's over speak with Varka and Gren. They are entusiastic about the fight, and they seem to have made peace with each other. The mission completes.