Thassarian, aboard the Skybreaker, wants you to collect Orbs of Illusion from Bitter Initiates at Aldur'thar and use them on 3 Dark Subjugators there.
The Cult of the Damned hold the gate of desolation, eh? No surprise there.
Time to have a little fun. At the southern end of the gate, you mentioned there were new cultists being trained. They're almost certain to be holding a special item given to all initiates - an Orb of Illusion.
These artifacts are used to disguise a cult member, allowing them to gain safe passage, infiltrate, or simply blend in with an enemy.
Kill the fools until you find one, <class>, then use it on a subjugator.