It was a prospering fortress. A rich one. More than 100 happy dwarves, without a single tantrum in years. There was a good amount of monster slayers living into. Everything was pretty much in order. Lots of food, clothes for everyone, golden furniture within, a barony, twenty dorfs on the military with a full set of bronze armor (mostly exceptional/masterpiece). Even a marksdwarf squad. Not legendary warriors, but the militia was in a decent shape after one year of training. Then, a Forgotten beast appeared. There was already another FB wandering the caverns. As soon as the new one enters, both start fighting. The new one slayed the wanderer in a short combat.
Then I decided that it was the time to fight the forgotten beast and claim the caverns for myself. As soon as I opened the cavern bridge I moved 20 hammerdwarves + 10 marksmen into the fray. There was about 4-5 monster slayers that came into. The FB was a gigantic hidrosaur. And it killed 30 dwarves in a row. There was no hope, every single hit was a dead dwarf (gore included). As soon as all of my military and monster hunters were dead (a pair of legendary MH included) the beast entered the fortress and slayed everyone in it's way.
I learned something the hard way. Don't mess with gigantic hidrosaurs. But it was a fun
And if in a future I find another FB like this, I'm sure i'll try a different approach.