Hahahaha just everything about this. Next: take over the HFS with creepy crawler husks.
Flying critters work best, ones that crawl on the ground are best suited for roaming tunnels and pipes where the demons' propensity for flying doesn't help them one bit.
All you need now is a minecart track set up to dump loads of corpses into hell with a necromancer set to raise them within... eternal siege by the undead horde seems like a grand way to hold off the forces of hell...
I do not yet have a factory IN hell, but I do have a factory in my dining hall Fortress for domestic processing of corpses and there is currently a factory which is as close to hell as you can get without being in it, being in the hallway that leads to the spire to hell.
The domestic factory is just a simple Necrobacon processing facility.
Tame animals go in, the bridge goes down covering the drainage stairs whilst letting the river flood the room (it also cleans corpses of any syndromes they are coated in, making them safe for Dwarf handling) and the necromancer reanimates them. From there either the weapon tunnel is opened for the corpses, allowing them to be riddled with bolts and processed in the butchery for meat or the cage trap tunnel is opened to harvest the corpses for war against the demonic filth.
There is even room to drop down corpses through the ceiling for reanimation.
And if a megabeast corpse is accidentally reanimated the Fort won't experience catastrophic flooding since all possible outflows are covered with drainage which would limit the flooding until such time as the rogue undead can be killed and the winter-freezing would allow repairs to be made.
It's a refined system that is almost entirely complete and optimized. It allows for handling the undead with as much safety as one can possibly expect from deliberately creating an army of muscle and bone opposed to all life.
The other factory on the other hand worked just as well; it just worked so well it's completely lost to the Necromancer Kogan.
It's overrun with the undead and full to the brim with the syndromes from 12 forgotten beasts and the HFS. It will take much work trying to take back control of the undead there from Kogan, but for now I'll let him have his fun as he's keeping the demons in a very dire situation. I could always run some water through it to clean the floor of disease (after all, the water will just drain into hell and piss the demons off even more) and might even flush some of the more stubborn undead into hell.
Watching the ravens eat demons is amusing me, though it seems that they are losing a few of their number to particularly resilient dung beetle demons which are also webbers. Losing ravens to the eerie glowing pits is also an issue, once my scout pig has revealed the layout of hell I shall pick a suitable site to begin building a Fortress, floor over all the glowing pits, build a great tower for a necromancer or two and maybe even wall off the entirety of the neutral side of the map so that the demons are always fighting on an evil biome where the corpses naturally reanimate even without necromancer prodding. Then it will be ready for true colonization by the undead and all of demonkind will forever be in a battle for daring to fight the Fortress.