Demons are a genetic offshoot of Angels. They are incredibly resilient and come in many shapes and sizes.
The origin of demonkind traces back to the First War, a civil war between two factions of Immortals. Those that lost were cast out of Heaven, doomed to be trapped in Hell, a ghastly realm cobbled together from damaged pieces of the universe.
The fallen remained in isolation for countless generations, but eventually adapted to their new environment. No longer true angels, they took on the name 'demon'. Hell itself was vast and barren, with many different worlds that slowly drifted even farther apart. As the generations crept by, new societies and subspecies of demons started to emerge and come into conflict with each other. But the one thing that they maintained in common was a desire for revenge on the angels that put them there.
Soon the demons began to advanced to the point where they could escape to Heaven, and the war between angels and demons reignited.