The Bloodskull Orcs are expanding their base of operations, Bloodskull Valley, and filling it with troops and massive war machines. This surely is a sign that the Bloodskull are planning to launch a massive attack on Freeport and/or settlements in The Commonlands. A brave adventurer is needed to ensure that this doesn't come to pass!
The objective of this instance is to burn the various war machines while slaying as many orcs and troglodytes as possible. If you haven't already completed it, make sure to pick up Terithal's Revenge from Terithal right next to the entrance.