The megalith was designed to deactivate the stasis locks on Endion that were keeping the Purifier space station Cybros in stasis in orbit. Without it, there was no way to deactivate the locks. During the End War it was activated to deactivate the stasis around Cybros, and the Daelaam protected it from the zerg that infested the planet. 
The megalith is an AI-controlled unit that appears on "Unsealing the Past." It will move on its own to each stasis lock and then spend a period of time immobile and invulnerable while it deactivates it. The megalith can attack, but is ill-suited to fighting the large number of zerg in the area, so the player must provide an escort to help it fight its way to each lock. If the megalith is destroyed, the mission is lost.