I'm considering giving up my umphteenth attempt close to the 1 year mark. Will have to reconsider the strategy once again.
Blood lying on the surface isn't fatal in itself, it seems. It's the falling stuff that's dangerous (possibly excluding small animals, see below).
Long story with no real info below, so feel free to skip.
The first part is actually easy, technically speaking. If you're unlucky, you're stomped on, and if you're lucky you'll make it indoors. I actually deconstruct the wagon immediately, since there's spare dwarfpower at that time anyway. I've dug down two ramps (old habits die hard; there are no trees here, but also, the HOT indication when setting up the pen/meeting area caused me to try to dig deeper, but it seems to be hot everywhere...), then two tiles in, and then a small room (6*6 or so). As soon as the first tile of the room is dug, that tile is designated as a meeting area and animal pen for the animals, and they tend to move on their own accord. It's then removed and reconstructed in a larger version as soon as the room size increases. When the room is dug a stockpile is set up, and I cross my fingers. There tend to be a number of times nobody seems to be willing to do any hauling (nothing hauled, 7 idle).
Meanwhile a civilian alert burrow is set up excluding all of the surface except where stuff to haul is, since I've had buggers idling on the surface. This is then reduced to exclude all of the surface and the tiles down, plus one tile of the two tile corridor when the hauling is done.
At this time you can breathe out, since you're out of harm's way for a while.
There is a cavern (I missed it on my first dig as well), but no moss or herbs, as indicated by vjek (but there ARE creepy crawlers).
I tried to actually dig a ramp all around the edge (3 tiles away, in case wagons need to get fully onto the map before starting to descend), with the intention to allow immigrants and caravans a chance to get below ground without dying, but that seems to be impossible without save scumming since the dorfs ignore all attempts to keep them below ground (traffic restrictions, burrows, dig UP RAMPS, DAMMIT! Nope. Us dorfs love the surrfass wif no horrible green stuff!
I then tried a more cumbersome version where only a single 3 tile wide ramp would be open at a time, using drawbridges to close them off. Unfortunately, the notifications only tell you when blood rain start, not when they stop, and with a map completely covered by blood there is no way to time it (considering reenabling DFHack's performance tweaks just to be able to see the rain stop). I didn't think my miner was exposed but he died in a trail of blood a bit further on...).
My most successful attempt yet (the one I'm considering giving up) just gave up on trying to help immigrants and just went for turtling. I changed my mind a little bit into the summer and built a little loop from my initial entrance (thus avoiding all surface exposure to my dorfs) with two doors (will keep non building destroyers out), and managed to get it ready just as the first immigrant entered the map and immediately died. Some of the batch actually made it though. I built a double 10 tile raising bridge hooked up to a pressure plate. Good theory. The big animals were no problem. Step on plate, continue in, splat. The undead dorfs, however, are FAST!!! Manual door locking got them, though, but it's cheaty. The small undead didn't trigger my plate (set to min weight, to no avail), and a peacock mutilated my "militia" dorf (pick, no armor) so he died of dehydration while resting and everything was brought to a halt by a no bucket bring water (hah) spam lock until he died. His undead self managed to climb out of the smoothed refuse dump hole late winter (i.e. now) to kill one dorf and probably send 3 other into dehydration slow death (where this rambling ends way further down).
Sometime during the summer I tried the cavern. Just two drawbridges and a door, and the report said a giant rat was about. Should be safe, right? Dig the last tile. Oops, a troll there as well, and the rat ran like greased lightning into the fortress. Managed to keep the troll outside, and sent my militia guy chasing the rat. Killed it eventually. Ok, set up a refuse stockpile for the mangled body (had read somewhere than mangled bodies don't rise). A while later an undead rat kills another dorf, before being put down (note to self! Mangled bodies DO rise!).
Autumn brought a caravan, but I didn't either have a trade depot or any open path. The merchant died within a minute (probably a lot less), while the pack animal survived surprisingly long. A large immigration batch arrived later and all dorfs survived. Two small animals (bird and puppy) just died on the way (heat stroke?). Since they were in the middle of the dorf pack, it can't be rain.
Got a mood and the bugger wanted a smithy and a metal bar. Ain't got neither, but a smithy was built, and the caravan did actually carry a bar of pig iron, so let him out. Better an undead on the outside than an insane-to-become-undead inside. Shielded by madness, he sprinted, picked up the bar, made it back, and produced... a goblet. Yay!
Finally go a critter break in the cavern: a giant cave swallow. Out and get some very important resource and send a horde of dorfs trying to brick up a part of the wall, and succeeded in creating an outside pocket, but a named giant bat suddenly appeared. Civ alert! Run! Everyone made it back in, and the bat was still outside. Just sit tight and wait for a new break, and rebuild the badly depleted store of blocks. Dorf has been missing for a week! Hm, yes, undead in the cavern, might be what gave the bat a name? The dorf entered an epic fight with a troll, and actually lost, but the troll's last breath of life was taken by an undead bat. The bat won't path to the fortress (the troll hadn't risen when I opened the path), and I'm not keen on sending a bunch of dorfs for slaughter. Cave-in? Nope. Can't do it there without being exposed. Possibly carve a fortification and pelt it with bone bolts made from my non existent stores of bone (made armor out of it). Could wait for the batch of soon to mature peafowl, though, or make use of their parents.
My engraver (and untrained militia guy) suddenly found dead? Yes, first dead dorf climbed up out of the pit. I think I've been able to throw all the dorf bits down again, but 3 more probably dying dorfs? Might be better to restart and set up an atom smashing garbage disposal system instead.
Conclusion: This is hard...
vjek came in while I wrote my (not so) short story. I was actually up to 23 dorfs after the second immigration wave...down to 21 "now".
Moods are a brutal issue, but won't happen unless you get immigrants. Sitting tight with only the starting 7 should work, given a lot of time (at the current rate it will take less than 100 years...). Risking the cavern and trying to recover caravan items can help a fair bit (mmm, I can SMELL the logs the caravan brought), but it's, well, risky.
Well, vjek, this brutal embark of yours is sure both interesting and frustrating. Good job!
With a mega attack threshold of 6 I wouldn't have let the migrants in, or at least have made a significantly safer entrance that could be locked quickly. As the surface (or cavern) fills with undead, it self propagates and gets very hard to clean up, and I'm not too keen on meeting undeath boosted dragons etc.