The Travill Nether Portal is ancient. It was buried underground for unknown ages when miners in Travill broke through the ceiling. Not knowing what the portal was, they ignited it, and thus, ignited the 2nd Nether Invasion. Seconds after lighting the portal, Zombie Pigmen rushed through, murdered the miners and quickly went to work, mining down for obsidian. They gathered a good bit, and escaped back to the Nether, where, now armed with new obsidian, they could make new portals and invade the overworld from anywhere. The war began again!