Base of Operations
With every planet they conquer, that world becomes a new base of operations for them to use as they spread their influence across the multiverse. At the moment there are countless worlds throughout the multiverse that they control.
Demons originate from a hellish dimension originally meant to serve as a prison for wicked souls. The atmosphere of this "Demon Realm" is inhospitable to outsiders; and will either kill them or corrupt them the longer they stay their.
They once served a roll akin to prison guards in their domain, punishing prisoners as they served their time in this hellish dimension until they were ready for reincarnation; they would perform this thankless duty for countless eons until the arrival of a certain prisoner. He would eventually conquer the realm, becoming the demon king Armageddon. After his prison break, he went on a violent campaign across the multiverse as retribution for his otherworldly incarceration; and under his influence, demons attacked and devoured the souls of mortals. It wasn't until he assaulted Mobius Prime, that he was once again sealed away; and his demon forces were scattered across times and space. With their leader imprisoned once more, they search for a method to break the seal.