Not all evil biomes reanimate. The three kinds of possible interactions are evil rain, undead, and evil clouds (which can sometimes turn even a kitten into an unstoppable super-zombie).
He said it. Evil biomes get up to two of the three possible interactions:
- Evil rain chooses a random material that can range from blood (mostly harmless but messy), to custom materials with random effects of mild to moderate severity.
- Reanimation creates undead with some small variations in how much of a stat boost they get. Reanimating regions will have access to the world stocks of undead creatures, so they'll appear at the map edge.
- Evil clouds are a custom material that either thralls/husks or has moderate to severe random effects, up to effectively being instantly lethal.
Random materials might be the kind (glob or dust, etc) that sticks to the ground or victims, so in the worst case, that kitten super-zombie will infect anyone near it that gets some of the material on them.
In 34.11, I witnessed a few weird cases where rain/cloud (or something) effectively permanently affected every land and air tile in the entire region. Birds would instantly fall dead at the edge of the map, regardless of whether it was raining.
An embark in such a region would end instantaneously on unpausing after embark. (which was pretty neat) I tried an area that was partially insta-death, and birds would again die instantly after flying into a tile that was part of the region. Sadly, I never figured out exactly what interaction made that happen, because I wasn't diligent in taking those save files apart.
I've just started 40.24, with a 5x5 embark that is a reanimating terrifying region on three sides, and the undead wildlife are accumulating like crazy in the caverns. There's a decent horde of undead crundle parts. The only long-term survivor out there so far is a very tough flying forgotten beast made of tiger eye.
If you want a good tough embark, back up your region directory and try a few different embarks until you land in one that's bad enough. If you see undead wildlife, then you should have reanimation. Otherwise, slaughter a few animals somewhere in the evil region and leave them at the butcher shop to see if they come back to life. Wait for the first rain/cloud and you should have a good handle on how much Fun it is!