Remember that "Terrifying"
means two things about a biome: that it is Savage, and that it is Evil.
The Savage part just means it's eligible to spawn a few more interesting creatures and plants.
The Evil part means the same thing whether it's Terrifying, Haunted, or Sinister: evil plants and animals, plus 0-2 Interactions.
There are three kinds of Interactions that an Evil biome can have: (or so I gather from decoding the world.dat file)
- Reanimation
- Evil Rain
- Evil Clouds
I think a biome is limited to only two interactions, total, but I haven't played in a while and my memory is hazy. Here's what each one does:
Reanimation: This is the familiar phenomenon of body parts rising as undead. The exact properties can vary a little bit by biome, since a typical game will have a few different undead curses.
Evil Rain: This is something icky that falls from the sky, blood, ichor, etc. It may or may not carry a curse. These curses range from pain and blisters to rapid bleeding out. I've never seen a rain that creates husks, though. That seems to be reserved for clouds.
Evil Clouds: These are the creepy clouds of icky stuff that crawl over the terrain, and usually carry a much nastier curse than the evil rain. These are where you get husks/thralls, which is a much nastier curse than plain undead. The material it chooses for the icky stuff determines whether it sticks to the thralls and they can carry it with them and infect victims outside the cloud. (Which is one of the nastier things the game can throw at you.)
If you carefully mess with the world.dat files, you can manipulate these curses to put the desired level of evilness in your biome. That's insanely tedious, since no one's completely decoded the file structure.
I've seen one evil Interaction a couple times that I can't artificially reproduce: biomes that instantly husk/thrall any creature that enters them. Birds fly in, and whether or not there's a cloud, they get thralled, no matter how high they're flying. Dwarves wearing boots also get instantly husked. Maybe it's a material type that floats in the air, or becomes a property of the tile?
I'm not sure how that last effect happens, but it's one way you can have the game end instantly on unpausing on embark, as all the dwarves get thralled on the first tick, and you go straight to the defeat screen. As far as I've seen, that's the fastest way to lose.