The Dwarves were going about their usual business of magma working, quarrying and sacrificing pigs to the magma sea when they came upon a chance discovery - there was a second adamantine tube underneath Silentthunders. In the first tube 6 layers were excavated and nothing horrible emerged from it. The plans to dig out the first three layers of the second tube were cut abruptly short when the miners pierced the tube into a hollow cavity within.
We had dug too deep.
We had reached hell.
It was inevitable.
We were not without defences, the miner who had chanced on this tube had enough time to climb up the stairs and bat the hatches shut. I figured if I was stuck underground for over 400 years, what would piss me off the most in the world? Doors. Doors everywhere. Predictably the demons were angry and spent much time tearing them down in a rage.
The entire Fort was assembled in the approach to the Overworld, ready to face every stray demon that managed to make it past the first line of defence. Due to the unexpected early arrival of the demons the defences were not complete, not a single steel spike had been moved into position in time. That did not mean Silentthunders was unprepared. For starters there were 148 soldiers ready to meet their horde, and if the situation was dire enough 60 war pigs and eagles could be brought to the frontlines.
Another trap also lay up the Dwarves' sleeves, a dozen horrors of war, forgotten beasts lying in ambush beyond the door. Both sides smashed down the barriers between them unaware of their opponents on the other side.
The Forgottean Beasts crashed into the demonic horde and all but three were killed almost immediately. Two were brained in one hit. In return they injured just two demons and killed none. A sad showing for the Fort's favoured foe. But this didn't matter, the Forgotten Beasts weren't the army defending Silentthunders, they were just the reinforcements.
A captured necromancer called Kogan was allowed to redeem himself in the eyes of Dwarf kind by ensnaring the demons in a horde of foul undead; the fallen beasts paved way for the true threat that has plagued Silentthunders since time immemorial.
RAVEN SWARMS!
NOT EVEN BADGERS HAVE CAUSED ME AS MUCH GRIEF AS RAVENS!
Kogan, safe from fire and syndrome within his bunker dutifully ensured that for as long as there were corpses there was a fight. The demonic hordes clawed their way through whole swathes of corpses to their own undoing, every step towards the Overworld was another batch of undead behind, in front of and beneath them.
As more demons began to fall to their rotten beaks the demons were less and less capable of destroying them. To be sure, whenever a corpse died many bodies were incinerated or melted before they could be reanimated, but many demons were also falling and they certainly weren't getting back up again. It got to the point where demons were incapable of doing nothing except try to break the grips of the hundreds-swarm of ravens to no avail.
They fell one by one and the undead remained resilient and angry until finally...
Only the sound of roiling undead remained. The corridor is irreclaimable, there are too many undead and although I can just stop them from reanimating by raising the bridges surrounding Kogan to block line of sight the entire chambre is full of the syndromes from 12 FBs and an entire clown car full of demons. Some of the undead FBs may still also be alive and ready to turn the trap into a gas chambre once again.
The flocks of undead ravens have since begun spilling into hell, but that's the demons' problem and not mine until I try colonizing the place. I'll probably have to use the first spire as a new entry point, the second is too hazardous.