Blood War (first part)

Military action

Demons wage war on Devils and conquer Underground.

A group of Demons left on normal plane discover Underground. The conditions they find there are similar to those in Abyss. They return to inform their king and a decision is made to conquer the place and make it into the base/city of demons to be used to facilitate further invasions.   The war between the original inhabitants, Devils, and the demons lasts for thousands of years. Demons end up succeeding in conquering the underground and banishing what remains of devils.   But the war isn't over... Blood War (second part)     Blood War is often considered a legend rather than a true historical event.