Thassarian aboard the Skybreaker wants you to go to Aldur'thar and defeat Overseer Faedris, Overseer Savryn, Overseer Veraj, and Overseer Jhaeqon.
Based on your report, and my knowledge of the Cult, they have an overseer managing the operation of each encampment on the gate.
While taking their leaders down will only be a temporary setback, it will at least slow them down and keep them off-balance. And that, <race>, is a worthy enough goal for now.
Seek out these overseers and return to me when they have been dispatched.