Here is a heads up for those who think their Evil biome is 'safer' than others.
I embarked in another Terrifying forest, as I've been doing lately. I immediately begain building walls, digging down, expecting zombies at any moment. None came. I was dissapointed with this. Without zombies... where was the Fun?
I decided to continue on, figuring I could at least make a fort easy enough and get some exploring done of the rest of the things the evil biome had to offer. Then I got the alert: "A cursed ash cloud drifts torwards you!" I looked up to see what looked like a red miasma. Far from the walls of my fortress. I was intrigued, but not enough to let my dorfs aboveground.
I did leave my pack animals above ground to nom on grass... more on that later.
Eventually, during one of these ash incursions, I noticed a thrisp man seemed to have become a zombie? That was odd. I checked the unit list, and it said he was 'opposed to life' and a 'cursed ash thrall' I see... now the plot thickens. Better make a larger enclosure of walls to put my pack animals in.
Finally, a ash cloud spreads smack over my fort. As I said before, the top was sealed off with a forbidden, tightly sealed floor hatch. Good thing too, it turns out. Because all of my pack animals turned into ash thralls. Yep, the ash spreads right through walls no problem. Then I get migrants! Through burrows, luck, and unforbidding, I get a few of them inside! But the ones that didnt make it? Become more ash thralls...
Well, this is a bad trend.
Sure enough, when the caravan arrives for the first time, the entire caravan is slaughtered. More ash thralls! This continues... every migration wave and caravan swelling the ranks of the thralls. But I'm busy below the surface, working on making a renewable underground wood source... My magma furnaces glow with the fire of the deeps, I locate and am about to try to get at some adamantine for smithing... I will retake the surface!
Or so I thought.
One migration wave manages to decently swell my ranks. I get a group of ten dwarves for a military finally... and then I start getting a bunch of cancellations... apparently one of the dorfs to get inside safely, a Dwarven Child, was tainted. He only turned once safely inside my fortress... past all my traps. I send my military after him... They start chasing him around for a few minutes... then suddenly they all turn into ash thralls. While still in the heart of my fortress.
You can guess the rest, right? I'm about to re-embark now.