It's a forgotten beast. I thought/hoped it was a titan, a specific FB is way less likely to show up at my fort (though it is a smaller sized world).
Actually, it might. Creatures can attack your fort no matter where they come from. Bastiongate killed or captured every single FB in the world.
......
I.......
have a sudden need to make a fort whose sole purpose is to create a zoo (complete with gem or glass windows) of as many megabeasts, Forgotten Beasts, etc as possible. Stated goal is to capture: Killing (or allowing) even a single (on-Fortress map) Forgotten Beast is considered a tragedy.
Dwarves are replaceable. Forgotten Beasts are not.
......
no, not just Mega/FB Zoo.
ALL OF THE THINGS zoo.
Construct a tavern around the containment cells, and create a system that periodically feeds a captured goblin invader to a randomly selected beast. Surround the containment cells with fortifications and windows, so that the visitors can see the beasts but the beasts can't shoot anything at the visitors. I'd also vote for hall of legends if this gets built. Once you have captured every beast in the world, pit them all together in an epic arena battle and murder the survivors, so that there are no more mythical abominations to haunt the world.
You're thinking too small.
Once all the Beasts are captured, the fort is sealed shut [, a back-up save created for posterity/fun/community shenanigans], HFS is broken open (if it hasn't already), Dwarves are outfitted with the finest armor, weapons, and training that Dwarven culture has to offer, and the Zoo's built-in safeguards are disabled--releasing every beast into the Fortress in a free-for-all.
May Armok have mercy on their souls.
.....somewhere a Vampire or a Were is sitting with a small farm, preferably in Fire Clay, with a Still, and minding the Bookkeeping. Keeping track of every toe, fingernail, and scrap of skin that gets torn off in the melee(s).
Then, when the dust has settled, and only one or two beasts remain, a door is opened, and the Champion is unleashed upon the surface--and the world.
Then, when the creature [ s ] is/are away, a lever is pulled, and the entire damn Fortress falls down into Hell.
......I kind of want to write that story in the style of the Increasingly Tragic Tale of Dumplin.....
EDIT:
so.
If I want as many creatures as possible, I will need a Good/Evil border, preferably with high Savagery. hmmmm.......time to poke around the worldgen section of the wiki.
so far all I've done with worldgen is turn metals to like 4-500 (YOU get a metal. and YOU get a metal. look under your chairs, EVERYONE gets multiple shallow and deep metals, with Flux!), turbocharge the number of volcanoes, and dramatically scale up the number of megabeasts in the world.
......Good/Evil border with Savage Mountain (For Yeti, and because wiki says Rocs live in Mountains) tile. Also preferably a Savage Tropical for Elephants, etc.
........or I could go for Sasquatches instead of Yetis? ......hmmmm........