Felhunters are magic eaters that feed on arcane energy, an energy that fel energy is counteracted by. The volatile arcane energies—the lifeblood of a nascent titan— are the energies of the Twisting Nether, an astral dimension separate from the physical universe, and so these hunters presumably reside in the Twisting Nether, searching for arcane energy to feast upon, or travel in-between the Twisting Nether and the physical universe.
In the face of demonic power (or warlock magics), most heroes see death. Warlocks see only opportunity. Dominance is their aim, and they have found a path to it in the dark arts, the arts of necromancy. These voracious spellcasters summon demonic minions to fight beside them. At first, they command only the service of imps, but as a warlock’s knowledge grows, seductive succubi, loyal voidwalkers, and horrific felhunters join the dark sorcerer’s ranks to wreak havoc on anyone who stands in their master’s way.