During the Second Age, for hundreds of years, the Zombie Pigmen and their Blaze Overlords waged war on the Realm of Vasten. Elves, Dwarves, Treents and many more races all worked together with the Humans to fight off the Pigmen, but it was a losing cause. After generations of war, the humans sent in small groups to break the Pigmen's obsidian Nether Portals and destroy the obsidian in lava. Given that water can not exist in the Nether, the Zombie Pigmen (and the remaining humans who broke the portals) were banished in the Nether.
For thousands of years since, these stories have become just stories, until one late afternoon, when in Travill, miners relit a portal and gave the Zombie Pigmen access to obsidian to once again wage war on the Realm. This current attack is known as the Nether Invasion.