The phoenix. Or, as its acolytes would say, Lord Bahu, the flaming wings, son of the sun, the last judge, the voice from the glass desert, may the whole world bask in the light of its glory.

Although it's often represented as a flaming eagle, the actual shape of the phoenix is unknown. The few accounts by people who looked at it and survived (its priests) describe it as a point irradiating massive amounts of heat and light. It's usually located at the heart of a massive refrigerated brass fortress that spews a continuous column of steam.

Most of its powers are pretty straightforward: it can melt every known material, vaporize cities in half a second, blind or kill anyone who dares just looking at it... But it can also turn the land sterile so nithing grows on it again, make the children of whoever approaches it deformed for generations (gifted or marked by Bahu)...

It can also see and act through its clergy. Every year, a new promotion of seminarists is tossed into the phoenix's chamber, and a handful of them manage to come back outside as blind, charred fanatical clerics. They can not only communicate with their lord, but also channel all of its powers, at the cost of turning more and more of themselves into ash and fire. This clergy manages an ever-expanding empire and their legions.

The prevailing theory among western scholars is that it must be some sort of weapon left over from a mythical war between the gods (well, the guys they left in charge) and the ancestors of all sentient races. But no one knows for sure.

And yes, if you read carefully, part of it hints at some kind of self-contained continuous nuclear explosion.