I've got a problem, it seems. I have a save shortly before a huge goblin siege shows up (it's fairly consistent, so far). The siege contains the goblin leader, a general, who just so happens to be a one-eyed Lizard Fiend with a deadly sting (Ronad Dankcackled the Drip of Stretching). He's wielding several pieces of dwarf-tooth and dwarf-bone jewelry as weapons. I haven't seen him sting anyone during testing yet, but the raws indicate the sting would cause massive ZZZs. Legends tell me he's been leading the only surviving Goblin civilization for over 1050 years, and has racked up 82 kills to his name (all dwarves) and led 51 battles. In fact, he's the deadliest creature in my world, as the second-most-deadly creature only has 10. During his travels, he has tamed a Rutherer, Jabberer, Blind Cave Bear, Cave Dragon, Cave Crocodile, Giant Toad, Giant Olm, Giant Bat, Giant Rat, and Giant Cave Swallow - all of which the goblins now use as mounts.
I've got lots of resources, a strong military and decent defenses; it'd be all good if I just wanted to kill him, but I read that the goblins will cease being mounted in future attacks if I do. How can I capture the demon? He's trap-immune, mounted, and going to show up with 100 of his best goblin buddies. I don't have a GCS yet, having avoided the caverns so far. I've never done any kind of cave-in (on purpose) and explanations I've read were hard to understand for some reason. Can someone please explain an example setup to me as if I were a small, simple child?
Even if the siege/general doesn't show up soon, by random chance, I know he will sometime. Since I've saved it before the siege even arrives and I'm not opposed to save-scumming, I have some time to plan or build. Ideas please?