Uploading for bug report.
A group of Raven eerie murk thralls has entered the map, and killed one of their own. They will probably start killing everything on the surface soon. Starting seven are walled in underground with food supply.

Copied from report:
"I embarked on a location covered by two neutral-aligned biomes. After a few months, I noticed I had a group of "Raven eerie murk thralls" on the map, attacking and killing each other (is this normal for thralls?). They each had "eerie murk covering" all over their bodies. They attacked my pastured livestock, converting them into thralls which also fought each other. Those in turn attacked the dwarves in my totally defenseless fledgling fortress and killed them all, ending the game.

There was no sign or announcement of any evil weather on my map, so I believe the raven thralls entered my map like normal wildlife. I lost my save when the fortress died, but had a backup from the moment of embark. Letting it run for a season had the raven thralls show up again, so it's reproducible on this embark anyway.

Here was my embark location: http://i.imgur.com/FrLyIpk.png
The nearest evil region is a haunted marsh to the southeast, which is two diagonal tiles away on the "World"-scale embark map."