I smiled
I'm glad you liked it. That joke was, like, 75% of the reason I posted in the first place.
Well, it wasn't as spectacular as I'd hoped it would be, but it worked well enough.
I designated an arbitrary chunk of the caverns as a tavern to encourage the siegers to path there. Trouble is, I forgot about all the "visiting" undead spies. They pathed towards the meeting area much more quickly than any of the hostile ones. So the Forgotten Beast wound up fighting them in a cramped tunnel where its wings meant nothing. And because Intelligent Undead don't follow the same rules as regular undead, the visiting necromancer had no trouble bringing them back into the fight no matter how many times their heads got pulped. The forgotten beast only got one kill that actually stayed dead before the revenants knocked its block off. Funnily enough, it earned itself a truly appropriate title during the fight: its full name was Sodor Tombsdied the Dead. May he rest in pieces.
So now I have access to the caverns again. In addition to gathering as much wood as I can, I'm working on sealing off a section of the caverns for fortress use. Wouldn't want to be caught off guard by another forgotten beast. And I'm gonna need to be able to access the surface again anyway, so I may end up rerouting as many undead as I can into the caverns. Or sending another forgotten beast up to the surface. Who can say?
Edit: Wouldn't you know it, I was caught off guard by another forgotten beast! I managed to get the caverns walled off in time and opened up the tunnel to the surface. It got killed by one of the twice-undead gaunt stalker spies. And, fool that I am, I ran this tunnel through the reanimating portion of the map. So now there's an enormous undead thrips spewing its noxious secretions all over the tunnel.