How do boss death actions work in UMAPINFO?

When some enemies die, they call BossDeath as an action: Baron, Cyberdemon, Spiderdemon, Mancubus, Arachnotron. You can use the bossaction line in UMAPINFO to make something happen when all enemies of this type die. If you want other enemies to call BossDeath, you need to edit them with Dehacked so that they do. If you want an enemy with a gibbing death to call it when they die, make sure to use it in both the normal and gibbed death frames.

For DEHEXTRA actors, you use the thing names Deh_Actor_145 through Deh_Actor_249.

A "line special" is just a linedef action, by number. It's easiest to just use a map editor to look these up, e.g. if you want a door to open, look through the special actions for doors, find one that works such as "61: Door Open Stay" and then use that number 61 as the linespecial argument. Then put a tag on the sectors you want to be affected and use that tag in the tag argument for the bossaction line.

Other than exits, I've found that only switch-type line specials (S1/SR) work in all ports.

Here's a thing I made that uses UMAPINFO to end the map when everything is killed. While it's probably not the effect you're after, it might help you as an example anyhow: