A black reaver
From EZ Server Wiki
A black reaver is part of the Caster's Guild Quest.
Loot
Tips
Black reavers can be found in the locked castle area and can trigger a random number of additional black reavers to chain spawn once it dies.
Once you kill one, there is a random chance that another black reaver spawns immediately in its place. Each reaver has the same chance of spawning yet another one, so it is possible for a chain of many reavers to be killed from a single pull. They have a 2 hour respawn time, so it is recommended that you use instances and repop the zone.
After the fifth or sixth [can be much more than 6, have had double digit reavers before] Lord Rak'Ashiir can spawn. No further reavers will spawn after Rak'Ashiir.