This prison is famous for its rumours: the Defias are said to have held high-ranking nobles there for ransom, and it was even rumoured to have held King Varian Wrynn himself. In Classic you could visit the island but never go inside. Now you can, and the Defias and naga are fighting within: an opportune time to move.
Blizzard hasn't published the bosses yet. We'll add them as soon as they're known (the beta starts on September 17).