Complex sentence: They kill Elder Nihlathak for granting Baal access to the Worldstone Keep and the player gains access to the keep, fighting Baal in the Worldstone Chamber after defeating his pack of minions at the Throne of Destruction.
Split into simpler sentences: They kill Elder Nihlathak for granting Baal access to the Worldstone Keep and the player gains access to the keep. After defeating Baal's pack of minions at the Throne of Destruction.