An unresolved conflict, going on for centuries.
Demons, invading the Divine Sanctuary.
Angels technically started it....Though, the aggressors are te demons instead. Angels protect, rather than attack.
Even so, a majority of the demon-population, gets used against their wills, to contribute to the war and potential death of their souls. Even angels aren't safe from attacking ine another;
Powerful demonic entities, can make them turn against eachother. A specific family of them comes to mind, who are responsible for it. Angels aren't actually sure, how to fight said control. They still are working very hard, to figure out that trait of theirs.
The war started, by the princesses of peace and love, not wanting to marry the princes of chaos and destruction. Now they are known as angels and demons, due to their nature being coined into said words, by a population in the billions; of humans and mobians, all labelling them so, be it with religious intent or not; That's how their names ended up being.
Some of the princes were so heartbroken, they wanted bitter revenge. Pride and Lust, specifically, though Sloth and Wrath were lesser affected than them, but also denied a marriage. Their brethren, Sloth and Greed, were very upset to hear it from them. While Sloth was consoling them, they themselves, were hurting deeply. Greed, trying to go past their grudges of materialism, offered the following; "Why don't we hurt them, just as much as they did us? They deserve it! It's only just, as they are bewitching. They stole your hearts...Steal them back. Eradicate them. Take back, what's ours!", Envy joining in on the plan, in their purely petty nature, added; "They banished us away in the FIRST PLACE! We used to be PART OF THEM! WE CAUSE BALANCE IN THE WORLD AND THEY DO THIS TO US?! WE GET TO LOOK HIDEOUS AND THEY GET TO BE WORSHIPPED?! BUT WE...?", as Wrath joins in.
"We WILL make them pay, show how powerful we are, out spirits WON'T BE BROKEN. Brothers, we will rise..And oh how extremely we will. I have a plan, but we need our most powerful magicians and witches, located in our domain, sent here, sharing the same thoughts...Banished, to a place, we don't deserve to reside in. They live luxuriously, up above, while we endlessly suffer below. Things will change..We will."
The siblings started all to agree slowly more and more, with one anothers's points. As they planned and talked, promised higher positions of powers, to those, willing to fight for their right, to live anywhere they want, instead of being banished, hidden away from the worlds, recruited a few. The may family agreed to find many more, the Crux family, gathering their most envious of abominations. Pride, having an entire army behind them, before even the war, giving them directions and lust..Twisting words, toward other heartbroken and seperated lovers, to empathise with them, to help, pleading kindly.
They were ready. And ever-since, there were never-ending wars.
Demons would especially attack, via creating summon-circles that lets them into any universe, place or time. They would invade the Divine Sanctuary, just to drag more angels, down to the underworld with them. They built an army, by making an innocent turn against oneself, transforming them, with their infectious blood. They fed off angels, like parasites.
It caused the whole angelic realm, to retaliate in return, with further seals, lineup defenses and a desire to protect those, who do not deserve the Princes's endless spite. The war has been going on for many centuries, between generations of humans, seedrians, aliens and mobians. The gods try to protect their folk, while the demons slowly more and more rot them out. Their defenses will eventually break. More and more angels have fallen, though also their armor and weapons increase in power. It's a race against opportunity and time. Which side will come out victorious?
Only time will tell.