Four score and seven years ago, our fathers brought forth - wait, no. That's the wrong thing. I am so sorry. Let me start over.
Eighty-seven years ago, the Fifth Dungeon of Trials, the Red Core Mrota, went insane and began sending its spawned monsters into the world, destroying everything in its path. The three kingdoms which bordered its domain sent their "Delvers", teams of skilled dungeon-clearing adventurers, to hold the line against all the horrors Mrota could produce. Five of the greatest Delvers, now history's most renowned, were called to the bloodied trenches from the corners of the earth to enter the Dungeon Mrota. They were given these instructions: clear all one hundred and twenty-two of its floors before placing a magical explosive on its core: an explosive which could be detonated safely from a distance, eliminating the threat it posed to the world's sapient species.
Alas, no plan survives its implementation, and the five heroes had to detonate the explosive from within. Mrota was destroyed, and the heroes perished within as the demiplane imploded. Mrota's magical energy, its "Ether", was released from its Gates violently, leaving a dead zone ten miles wide. A statue was placed at the center of the crater where Mrota once stood - five pillars of gleaming brass to symbolize the five who had fallen.
Eighty-seven years have passed, and rumors have begun to spread. The crater is no longer safe. Traders that pass through, hoping to cut their routes short, go missing. There are whispers of strange plants, mutated creatures, abominations prowling the roads at the border. A disease is spreading through the population like wildfire, an incurable disease that leaves the victim dead within days - if they are lucky. The unlucky are broken, their minds twisted by otherworldly visions and hallucinations.
But not all hope is lost - a new Dungeon has been born; a new kind of dungeon. It will become a defender of the natural world, a bastion against ecological collapse, and a foil to Mrota's revenge...a "Green Core."