Cyrus the Black drops Tol'Vir Hereditary Girdle, which is a bind on equip item level 308 mail caster belt. He also drops around 20 gold, and for leveling characters killing him yields a hefty amount of experience.
For leveling characters that come across him I would like to advise that you be careful when pulling him, as it is easy to aggro the adds around him. He can be pulled without aggroing them.
Like Armagedillo, I haven't done a lot of research on his spawn timer, but I can tell you that he is up quite often because on my route looking for Mysterious Camel figurines I have killed him several times.
He is also one of five rare spawns found in Uldum. All of them are:
- Akma'Hat <Dirge of the Eternal Sands> - Group Boss, drops a nice epic belt
- Armagedillo (HUGE armadillo) - drops a 2-handed mace
- Cyrus the Black - drops a mail caster belt
- Madexx (Tamable Scorpid) - Spawns in 5 different colors, only drops a tear
- Mysterious Camel Figurine - takes you to Dormus the Camel-Hoarder who drops Reins of the Grey Riding Camel