Overlord Sunderfury drops Sunderfury's Sundries, which are bind on equip item level 345 cloth pants. 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. It is probably best to pull him toward the edge of the cliff if you can.
His spawn timer is probably similar to other Cataclysm rares, but I personally have only seen him up once and had to actively look for him for quite a while before I finally found him up. This particular rare can be pretty hard to find up because of the location that he spawns in. The odds of him being noticed and killed by someone questing in the area are pretty high. If your looking for him, I advise checking early in the morning when less people are on. I found him around 7am eastern time.
He is one of five rare spawns in the Twilight Highlands. All of them are:
- Julak-Doom <The Eye of Zor> - Group Boss (NOT a pushover)
- Karoma (Spirit Wolf) - For BM Hunters, Only drops a tear
- Overlord Sunderfury - Drops his pants
- Sambas - Tamable Lion, only drops a tear
- Tarvus the Vile - Interesting looking purple dragonkin