As a quick question, what Divine Rank would a party of 5 characters, backed by their armies have to be in order to siege the Iron City of Dis and unseat Aeshma-Deva?
Well in any war the defender probably has a 2:1 or 3:1 advantage, Devils being organised leans toward the latter. That's the first reason Divine Realms don't get invaded that often.
The second reason is allies. So if an Immortal (not of Hell) invaded the realm of an Arch-devil, ALL the Arch-devils would band together to support them. Not to mention other local Hell-resident immortals (Druaga, Hecate etc.) might also pitch in seeking favours down the line.
So realistically you would probably need a party of Greater Gods to unseat one Arch-devil and conquer its Realm...at which point Hell would probably start taking the threat as existential and release fallen Arch-angels like Lucifer, Aeshma-Deva etc. Or using rituals to summon beings which would put up more of a challenge to repel the invaders.
Also you would lose most of your Army in the invasion and thus open yourself up to Counter-Invasions from your enemies who would likely invade YOUR Divine Realm; meaning you'd be facing a war on two fronts - never a good idea.
Added to which you would be largely punching down (Greater God invaders vs. Lesser God Arch-devils) so Hell would be fully justified recruiting some Evil Greater Gods as mercenaries to aid them.
Hell is also likely to strike against your worshipers.
Plus even if you did somehow conquer Hell it would probably corrupt you eventually and turn you to Lawful Evil.
...so its probably not a good idea.
