He Who Will Swallow The Sun/Moon/Earth

Long ago a civilization of sun-worshipers with little practical knowledge of astronomy witnessed a total solar eclipse, and over centuries the trio of serpents were born from their fear. Suneater was born from the original eclipse, though he was not particularly powerful. Moonbiter was born centuries later from a different civilization, during a lunar eclipse. 
The two serpents knew that there would be a third, and that there would come a time when the three of them would at once swallow the earth, moon, and sun. The End was born when an astronomer managed to calculate the day the next solar eclipse would occur, and the information made its way to the local doomsday cult. Unfortunately for the trio of serpents, a god of chaos briefly caused the sun to rise in the south and set in the north, and all astronomy-based prophecies were nullified.
Suneater refused to change his plan and declared they would end the world with or without an eclipse. His reasoning was that the sun was the sun whether mortals could see it or not. Moonbiter disagreed, and insisted that they could only complete their destiny during a solar eclipse. His reasoning was that only the appearance of the sun being 'swallowed' could allow such a ridiculous thing to happen. The End was willing to follow Suneater's lead and waited for his signal, but Suneater never returned. 
Moonbiter realized that with Suneater dead they could never succeed and gave up, existing as a god of the lunar eclipse. The End did not give up, but with his brothers out of the picture he had no reason to stick to a schedule. He climbed the World Tree and entered the Astral Plane to wait for his moment.