Use Cabal Gold to socket a Challenger Medallion into the Hammer of Proving. Once done, you will be ready to issue a challenge. The Hammer of Proving can be found in the Quest tab of the Director.
During your mission to foil Ixel, the Far-Reaching on Nessus, troubling data pulled from Vex prediction engines foretold a series of futures where Zavala has suffered a final death. Crow believes the Cabal will try to act on these predictions and that Zavala is in danger.