Kil'jaeden's Seekers are demons commanded by the demon lord Kil'jaeden. Not much is known about them, and only one member appeared, an infernal, on the Broken Isles after the Third War.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.
The group could be seeking out artifacts (the naga were uncovering a few during the mission) for their master. The infernal could've been sent by Kil'jaeden to assist Illidan, providing an early clue that they're allied.