Instructions
Begin the Alien campaign.
Play through the Labs, Colony, Refinery and Jungle areas until you reach the Ruins, the final stage.
Activate the three floor plates by dropping down the central dais from the outside. The first one is hit when you land in the water, and the next two are found in the pool with the alien statue and the other pool on the opposite side of the canal. This unlocks the human enclave.
Destroy the power source in the human enclave.
Head for the arena, which is now accessible.
Battle the predators. Reduce the Master Predator's life until the "Harvest" option appears.
Harvest the Master Predator and watch the campaign ending sequence. Using the Master Predator's body as a host, the larvae has grown into a new kind of Alien Queen.