I'm wondering if this is related to what happened to me today.
Just started up my first fort in over a year, spent all afternoon carefully choosing the site and preparing my embark... Picked a site straddling a temperate grassland and a temperate savanna, both neutral. Had just managed to get the first few levels designated and the bedrooms/future tavern carved out, when I suddenly got a combat report showing up.
A big group of "eerie murk thrall ravens" had shown up and were attacking each other. Inspecting both the live and dead ravens showed a covering of "eerie murk" on them...
Long story short, they attacked some pastured animals and thralled them, and then those attacked an outside dwarf and thralled them, and then they all killed each other until only one horse thrall remained. Since I hadn't been expecting anything like this I'd been putting off defenses... didn't even have any doors yet. Horse thrall marched down into my tavern and killed everyone, fortress over. Very sad.
The closest evil region was about 3-4 world-map-embark tiles to the east. I didn't see any evidence that an actual evil cloud showed up, though... I think the ravens just flew onto my map as wandering wildlife. I lost my save when the fortress crumbled, but restarted from an initial embark save, and the same raven thralls showed up again a few months later, so it's repeatable, and really not what I was looking for out of my non-evil embark...
Is this something that happens often? Should I put it up on the bug tracker? ...and is there any easy way to selectively disable evil weather after embark? I was really pleased with that location... :/