The Under Gates are commonly believed to be so named, because out of all the world, they possess the greatest number of natural entrances to the Unders, the vast network of tunnels that are beneath the surface of Corona.
This is a falsehood.
When the world was young, three gates were opened one after the other, and something crawled through. It sought to explore this strange place, and it left it's mark on the world, transforming it. With each gate it passed through, it warped itself to be more like the world. It also transformed the world less than it had before, for it found that every time it got closer to the world, it had made the world closer to it, and it was uninterested in exploring a world closer to itself rather than further from it.
The last gate it passed through was the Gate of Illusions, where it cloaked itself in the final visage of reality. From then on all who gazed upon it would know it to be of this world, and not other than it. It's final twist on the world was to hide the sun, and place a new one above it. The Sun did not appreciate this, and waxed wroth that a liar and a charlatan should have domain over any part of the world at all. It was this day that the Sun stood firm against the lies of the world, and so it became that light shines into darkness and reveals the truth. The being who entered the world was a master of the illusion magic it had invented by entering the world, creating the ability for others to seem other than they are. It was able to explore almost the entire world before the Sun, triumphantly tore the illusion of it's belonging from it's body, and made it's presence known to the gods. One god was so enraged by the presence of this being, that it changed it's very purpose from one of simple purification and aestheticism to that of guardianship, to insure the visitor could not threaten the world.