The Cracking of the World
The demon Vritra performed a ritual to release The Tarrasque. The ritual took place on a coterminous part of the plane of battle-Shavarath-located in the Howling Peaks while the PCs fought goblins of Darguun and warforged devoted to the Lord of Blades.
The seals holding The Tarrasque back break over the course of a few years. Each seal is located at the nexus of a different plane and Eberron. Each of the thirteen planes, plus Eberron, Syberis, and Khyber, contain a seal.
Sur’Kil claims that no force will actually stop The Tarrasque from being released, and that the PCs have only a slim chance of learning enough about it and preventing it from growing more powerful than they could handle.