In the upcoming version evil regions can gloriously spread outwords from necromancer towers. In adventure mode in the current version the player can become a necromancer the player can also use their zombies to build a camp. So will evil regions spread outwards from these sorts of camps If the player takes some time to build up the zombie population?
A while ago you talked about “really bad things” causing a region to turn evil (whether by sacrificing many creatures with a small age number, or some other terrible catastrophe. Is that still planned and will players be able to do this?
I'd expect the spreading of evil from a necro tower to be tied to the tower itself (and it being inhabited by a necro so it's active). If that's the case the question turns into one of whether an adventurer can turn its camp into a "necro tower" or if a necro adventurer can "turn on" a necro tower.
My guess is that the answer to the first question is no for the time being (too far off the center of the current development arc target), with the second one a "maybe" depending on how easy it is for DF to detect the necro's presence and activities.
Regardless, spreading evilness in adventure mode is going to be a rather tedious task given the rate at which it spreads, with the time scale being years for any shift and decades for significant effects to be seen. Sure, a necro has all the time in the world, but the player doesn't. It would actually be more "useful" to turn a player fortress into a "necro tower" when it comes to noticing the spread of evil, but I don't expect that to be possible in the Villains release.
For the really long term evilness spreading purposes you'd want epic time scales, such as a world observation mode where the activated world just continues while the player watches passively (or goes off to do something else, such as catching up on sleep or earn their living), but that's not something that's going to happen in the near term, although the embark time has been DFHacked to allow for that activity to progress much longer that the standard two weeks, but you'd need to make an embark every time you'd want to see what's happened since the last time, as well as to save the progress, and won't see what's happening in between these times.