Siosivash

Siosivash is an ancient demigod Primordial who declared war on the gods and sparked a millennia-long conflict known as the Dawn War when he cleft the god Io in two pieces, killing her, but also birthing the twin gods Bahamut and Tiamat.
After he was defeated, he was imprisoned in a special demiplane designed to hold him and his armies forever. This plane became known as the Upside-Down.