Ulrezaj had stolen a sacred khaydarin crystal from Shakuras, and while tracking it, the protoss discovered that Ulrezaj had created five twisted khaydarin crystals on Char. He was using them to control zerg minions.
Zeratul needed a dark archon to destroy the warped crystals, as only its psychic fury was capable of it. It was accompanied by several other heroes in the mission. The Dark Archon Hero could use its Mind Control ability to bolster the strike force's strength.