So, snagged Steam DF and set up a fort.
Set up a tavern. Almost instantly got a couple visiting necromancers and their undead companions. Did turn off invasions, so these were "friendlies."
I stepped away for about half an hour and did not see what happened next develop "live."
When I returned to the game ... TOTAL FPS DEATH, 0 FPS. Spacebar pause function did work, if I held down the key for a few seconds.
All had been well enough until the full moon hit and a wererat transformed and went berserk in the tavern. Everyone attacked the wererat, including the necromancers. The necromancers used their abilities in combat and resurrected my dwarves and fallen visitors.
Now, fights tend to get hit FPS a bit, but usually temporarily. I let the fort run for about an hour and a half, but things didn't resolve. I did notice the wererat, as the full moon was still out, and sent my squad after it.
Unfortunately, the spacebar pause stopped working, so I decided to let the game run overnight.
When I returned after about twelve hours, FPS was only 1, but the spacebar pause worked again, and about six weeks of game time had passed. And the fort had just "regained its senses." So, likely, sending the squad against the wererat had triggered either a loyalty cascade, or a tantrum spiral. Or it was just the wererat's kills that created the tantrum spiral.
Didn't let the fort run much longer to see if FPS picked back up after everyone finished calming down, or if all the splatter and bodies and body parts were still weighing FPS down.
So, finally retired the fort and explored it in Legends mode.
Found a few lines where one visiting undead, a gaunt slayer, kept resurrecting, maybe by itself, as there was no mention of a necromancer re-animating it. And several times! Not sure if any others had the same behavior.
I think the zombies self-resurrecting, and repeatedly, contributed to FPS death here. Or at least prolonged the FPS-sapping fight. Maybe there should be a limit on the resurrections in a certain time period?