Good progress. I have constructed a staircase to the slade floor without losing any dwarfs.(just some war jaguars were killed) I fixed the problems with the defenses so my military is safe. I keep checking the units page and evacuate when I see an underworld creature appear in the list, then wait for it to come up so I can kill it. Eventually, I should get the walls and fortifications up in the underworld so I can keep a permanent base down there. I'll dump a lot of booze and food to keep them alive, along with a a 100 bins of rock blocks to build a fort down there.
On an unrelated note, a cave dragon has arrived in the caverns. I set up some cage traps to try and catch it.
update: Yay! I caught the cave dragon!
Update 2: The dwarfs are butchering and eating Earthworm Fiend demons. They provide quite a lot of meat too.
I have a small defendable fort in the underworld now. It has 2 fortified mark dwarf positions to watch out for and kill incoming demons. One at the bottom and one at the top. They are pretty good about kill everything before it can get it. I boxed everything in with walls and ceiling and left one one entrance to act as a chokepoint for my markdwarfs to kill whatever tries to enter. I have a door to slow them down to be nice slow targets.
I figured out that I don't need a secondary staircase to perform the cave ins. I will setup a 2nd cave-in device inside the underworld, and funnel magma though the existing stairs, since fluids aren't impeded by stairs. All this means is testing is about to commence if I can dig through the slade floor.
At first it looks bad. I built over the magma flow that had collapsed down onto the slade floor, the removed the construction. This left a furrowed peat floor. I channeled it and it turned into open space. But when I went down one Zlevel to look, below the open space was just a slade wall. How disapointing. But. I haven't dropped the cave-in on it to see what happens.
Dropping the cave-in did nothing. Next, I am going to test what happened when I try over an eerie pit.
Its super easy to breach a 2nd staircase through SMR is you carve up/down stairs from the bottom to the top. It doesn't reveal whats above it, so its very easy.
One of the weaponsmiths got a mood. He grabbed adamantine wafers first, then a bunch of other crap, including gold nuggets. When he was done, he had made an adamantine war hammer. The perfect weapon for the hammerer.
Theres still a giant horde of demons crammed into the top of one of the adamantine tubes. I am going to build a skybridge across the top of the underworld lined Fortifications on the sides to shoot out from. Just to keep the dwarfs safe so nobody will dodge off the edge into an eerie pit or something.