There are several different types of "night creatures":
- Boogeymen: Only active in adventure mode, and only at night.
- Zombies (raised corpses of creatures [animals as well as sapients], but with no real intelligence.
- Necromancers. Sapient undead. Can pass as regular sapients.
- Vampires: Ditto.
- Husks: Created from creatures by evil weather. No real intelligence.
- (Failed) experiments: New creations of necromancers. I know little about them.
- Intelligent raised sapients: Raised by necros, but retain intelligence and some knowledge of their former life. Can pass as regular sapients. This it the type that went on a rampage, for some reason. They can come as visitors, as far as I understand, and so far behave like normal visitors, I believe, but they can also come as part of necro sieges.
- Ghosts: Spontaneously rising incorporeal former fortress members. Laid to rest through burial or placing a memorial slab engraved with their details.
Note that regular necro attacks can happen during daytime, and spontaneously rising undead in reanimating biomes don't care about time of day either.
Spreading evil does not spread to to settlements. If you settle in an area that's already under an evil influence that influence will not magically go away, but you'll know that at embark, unless you don't bother looking at all biomes you're embarking on (there there is also the possibility of incursions, i.e. small bits of a neighboring biome entering your embark tiles at the edges, if you embark right next to such biomes).