Door to Chaos (Chaos trilogy)

The Door to Chaos was the portal to Valhalla, guarded by a machine Goddess known as Etro and thus became known as Etro's Gate, Etro keeps the Gate sealed in an attempt to stop Chaos from escaping into the Realm of Light and turning Light into Darkness.

Trivia

 * Like Valhalla, the Door to Chaos is the same as Etro's Gate from the Final Fantasy XIII franchise.