This isn't quite a "war in heaven", more of a conflict between "immortals", but I was just working on it a couple nights ago so here's the cliffnote version of what I hope to weave into a longer story.

The First Gods of Pannos were immensely powerful beings. Their sorcery was magnitudes stronger or more complex than any other magic Pannos has seen. Their joy came from creation, and they created nearly everything on Pannos: The mountains, the seas, rivers, hills, flora and fauna. They took great pride in creating a complex ecosystem and tinkered with it over the aeons. Numbering 11, they were ageless, sexless, unburdened by the needs of food, drink, sleep, or sex. They had no concept of war - if First Gods disagreed, they merely isolated themselves across Pannos until their social natures became more important than whatever they quarreled over. Eventually one First God (Lyali) left Pannos to pursue new knowledge - losing them saddened the remaining First Gods, but they eagerly anticipated Lyali's return and the knowledge they'd discover.

Then one day the Dragons came. Hundreds of large, flying beasts poured through rifts of Dracomagic as the entire species left their home to migrate to Pannos. With prodigious appetites, the Dragons quickly hunted many of Pannos' largest creatures to extinction. While the First Gods were dismayed, they had no idea how to convince the mostly mindless Dragons to leave, so they simply repopulated their world to keep up with the appetites of the Dragon horde. Indeed, several First Gods enjoyed the challenge of an unplanned species being introduced into Pannos. That is, until the Dragons found the homeland of the First Gods and, true to ravenous nature, started hunting the First Gods.

The magic, teeth, and claws of a Dragon was no match for the sorcery of a First God, but the Dragons nearly 100 to 1 and the First Gods were unprepared to wage war (or "wasteful hunting" as they knew it). Three First Gods were slain in the initial clash, before the remaining seven First Gods started to craft their sorcery for offensive uses. Then, the tides of the battle turned and Dragons were slain by the dozen as the air crackled, burned, and froze by turn. The "war" had turned, as the Dragons had lost half their number within a dozen years and the First Gods still numbered seven. However, this wasteful hunting took an incalculable toll on the First Gods, for war was not a part of their nature. They discovered the emotion of hate, and finding that they had the capacity for hate terrified them even more than the threat of Pannos being ravaged by Dragons. They decided that their work on Pannos was not worth sacrificing the peacefulness that defined the First Gods, and having lost three of their comrades they were desperate to find and join Lyali, so they made plans to flee Pannos and search for Lyali.

They were not, however, willing to let the Dragons hunt Pannos' creatures down until it was a barren wasteland. So they made their greatest creation. Beings that would outpopulate the Dragons as the Dragons had outnumbered the First Gods. Beings created with war as part of their nature, that could not help but war. Beings smart enough to use these advantages against the Dragons, and intelligent enough to harness the magic that imbued most of Pannos. They created humans, and fled Pannos.