So long as we have had Gods watching out for us, there have been Demons who have been the enemy of all Mortals. So the Church tells us, anyway.
The Demon Realm is another plane of existence in which demons, the ancient enemy of mortalkind, live, rule, and spawn. Never contented with just their realm, they constantly ways to spread their corruption to other worlds, including ours, so they too may spread. Their world is one of dark magic, endless fire, and chaotic shifting of reality. A mortal lost their would inevitably lose their mind, should they survive that long. The plane’s form is at the mercy of the Demon Lords, the most powerful of their kind.
It is a common misconception that the demons and the Evil Gods are aligned, but this is not true. The ‘evil’ Gods see a different path for mortalkind than what we would consider true and righteous, a path of cruelty and greed in which each lives and dies for oneself and ones own power. The demons, on the other hand, view mortalkind as something to be subjected in the best of cases, and completely obliterated in the worst. They care not for the lives and souls of mortals, just for furthering their own power. This is why ‘evil’ gods may be feared, but demons are hated and considered the enemy.
The ancient Maenad of the Order worshiped the Demon Lords as gods. They created portals to the realm to commune with them, and where granted powers in exchange for helping spread their taint through worship. When the Maenad were defeated, it seems most turned from demon worship and sought power from different sources. Many traces of their demonic pacts and portals still linger in Absum.
Today, the demon realm only extends its power into our world in areas with extensive taint, or through the magic and pacts of powerful warlocks.