Taelkor
The Unmaker. The Black Architect. The only World Builder who refused the Awakening, kept his physical form, and spent the next seven thousand years trying to erase every living thing on the world.
The Church teaches the shape of it plainly. He held that the gods' binding to mortals was corruption, that free will was the disease, and that the only cure was an empty world. He came first with whispers and then with an army pulled out of the void. The Long War cost the world entire cities and bloodlines, and the Deadlands are the wound it left.
He was defeated at the Last Battle in what is now the Elfwood, his physical form shattered by a ritual of the greatest mages of the age, and cast into the void beyond the world.
Then, five hundred years ago, he very nearly came back, and the Heroes of Southaven put him out again. See The Second Banishment.
What He Told People
His recruiting pitch, recorded by the people who fought him, was that the gods were thieves. That they had stolen a world belonging to greater powers and hidden it, and that everything built on top of that theft was illegitimate.
Five thousand years of Order of the Black Star doctrine rested on it. Every priest on Newcome can explain why it is a lie a cult tells to justify slaughter.