My first fort to die in the new version(outside of issues with immortal undead and so on) died recently due to extremely bad luck.
Up until then, whenever a forgotten beast made of inorganic material arrived, I sent my military to fight anyway. They break all of it's limbs and the whole thing has body in the red? They won, use an utility to kill the thing.
What I didn't expect was that what would trigger the end of my fortress was a beast with deadly dust. A huge worm of some sort. It went down fast, but during the battle there was a lot of dust going around.
Turns out that dust made the soldiers bodies swell up with their own blood, while their bodies rotted both inside and outside. All but two soldiers died. Luckily I had kept my fort pumping out goods, and it seemed the survivors were too happy to be devastated by the death of all their friends.
But this still left me with a military of two and not seventeen which used to be moments before.
Migrants arrive, and the military is remade, twenty dwarves strong, weapons are handed out and so is masterwork steel armor.
Then the goblins arrive.
The civilians flee for their lives, the military stands strong. And that's when things went really bad. The goblins broke the backs of the military and the new recruits, and one of the two surviving veterans died when charging the enemy after the ranks at the gates were smashed apart.
The goblins did flee and only a few civilians died, but now I had a military of four people, two of them hospitalized.
Miraculously, a small migrant wave arrives soon after. All of them were drafted.
That's when Ost, the towering humanoid of horn silver arrived. So I sent my military to fight, again.
Things went bad again.
No one was able to put even one dent to the thing, I don't know if it was something off with horn silver values, or if my dwarves simply weren't strong enough(the two surviving veterans weren't even close to the best dwarves my military had).
Whatever it was, Ost took blows like being slapped with wet noodle, and one by one, punched my poor military to submission, crushing skulls and so on. And then he/she/it grabbed a pair of masterpiece steel greaves from one of my soldiers, and began beating everyone to a pulp with it.
And he did.
Every single dwarf in that fort at that point died because a horn silver humanoid grabbed a pair of steel greaves, and bashed them all to death with it. I think the defining moment was when Ost lodged the greaves in someone's wound and twisted them around.
So while this was a combination of things that went horribly wrong, I would have to say the forgotten beasts pose the greatest of potential threats a fort can face. If you're unlucky, of course. The beast before the worm, which also had deadly dust, only made blisters appear on the skin.
I might have gotten a tad careless at that point. It was still fun though.