I was attacked at the first autumn by a necro siege that was announced by a message I've never seen before consisting of human experiment creatures. After actually succeeding in bringing all the dorfs underground and locking the doors I received the message that the trade depot had been toppled, the the invaders were apparently building destroyers. Emergency wall building, as the doors would soon be brought down, and the were.
The invaders then left after just a couple of weeks after toppling and killing things, as well as bringing a giant bark scorpion back to life (must have been a hidden necro there).
It's now a year later, i.e. second autumn, and the buggers come in a 56 strong force. I've got some meager cage trap defenses that caught a hidden necro, but then I see that the monsters just run straight over the traps! Good luck I didn't actually managed to finish the emergency trap entrance last year...
The end result, though, is that there is nothing I can do against this early threat except turtling (and digging invaders is approaching fast...). My 17 untrained, unarmed, and unarmored dorfs would stand no chance at all (two seasons without migrants [not counting winter], for some reason possibly related to me just having been saddled with a monarch, despite trying to embark as a healthy civ), and two of them are already nervous wrecks, in one case because of being rained on by human blood.
I think (some of) the necros might have become a bit too strong in Fortress Mode (as well as in world gen)... It's approaching getting struck by HFS in year 1 if you happen to embark near a tower filled with these monsters rather than "this will be risky, as some necros can show up early".