Once upon a time, Triel was an Archon, a celestial creature of great beauty. Only, Triel cared more about Law than Good, to the point of becoming outright evil. Triel was cast from Mount Celestia, where he ended up in Baator.
The Baatezu embraced him, but transformed him into a creature like a gigantic slug or maggot, mocking his previous Celestial form. Even so, Triel triumphed over his rivals, ruling one of the deepest of the hells.
Even in his fallen state, Triel is consumed with perfection. His layer, once beautiful and green, has been reduced to ruins and slag as he consumes all of its natural resources in an attempt to build the most beautiful, orderly city the Multiverse has ever seen. Perhaps he believes that if the Seven Heavens will not have him, creating a heaven of his own, superior to the one he left, is an adequate riposte.