Quote Originally Posted by Eldan View Post
There was the War of Iron. When the Harmonium had pacified it's home planet of Ortho, they decided the next step to bring Harmony to the planes was to invade the Abyss.

They failed rather miserably.

But then, the planes were never a main focus for Ortho.
I think they were just trying to take war back to the demons who had invaded Ortho. They didn't understand what that "Abyss" really was, otherwise they wouldn't have tried something like that.