Happily almost a hundred greenskins, mounts and troll friends arrived just after my last post. I connected the caverns with the aboveground and the siege for whatever reason decided to head downstairs and visit my little guest where they encountered each other in a 1-tile-wide tunnel.
Ugsnor killed the first squad of seven or eight down the tunnel easily. A few of them succumbed to its poisonous vapors, which caused them to bruise all over and both eyes to immediately rot out. They ran off but shortly died from suffocation as they began to suffer full-body rot, including all organs.
Then the rest of the siege arrived, including a hammerlord and the goblin law-giver, an elite bowgoblin. They fought in the tunnel for a bit, but eventually Ugsnor decided to move back out into the caverns with all four feet removed but still very much alive and "kicking" so to speak. Most of the rutherer and giant olm mounts were easily dispatched by the rot. The goblins were a little bit better off, but a good dozen of them wandered off to suffocate and rot. Another two dozen or so were dispatched by Ugsnor directly. In the end, Ugsnor killed probably 40 or 50 goblins before a lucky(??) shot from the law-giver shattered his lower body, somehow, sending it flying away in an arc. (The law-giver incidentally died shortly after by the rot.) Some of the siege managed to retreat and run away, and the handful remaining were mopped up by my military.
So, in actual practice, a Forgotten Beast made of gemstone is a very, very dangerous critter. Add in the rot and it could easily overwhelm your fort. The best you can hope for is a really lucky shot like the one that took down Ugsnor.