At the end of this fort's second year, a pair of necromancers brought a small horde (only about 15) zombies to, y'know, spread terror and death, etc., etc. Due to a severe lack of armor and therefore of trained military dwarves, I decided to turtle up. Almost immediately, a were-elephant ambushed the only two zombies bold enough to approach the fortress and ripped them to shreds, before reverting to human form.
A few months later, a dwarf had a strange mood, and I was out of wood in the safety of the fort, so I briefly allowed surface access because the necromancers were just chilling out near the edge of the map...which they continued to do while dwarves walked purposefully around the surface, collecting all the logs left up there months ago and working on unfinished construction jobs. Since the necromancers had apparently just decided to go for a camping trip that happened to take place on the edge of the map and had no interest in attacking me (despite the big, red S I E G E warning), I decided to just leave the fort open and finish building the walls on the second story, properly connect the drawbridge (previously cancelled for no good reason, to my great irritation), and generally get stuff done.
This uneasy peace lasted until autumn, when they turned tail and left without even saying goodbye. I was taken completely by surprise by the caravan and diplomat arriving. Work continued, traded for some stuff, the mayor met with the diplomat, walls were built...and then, well after the meeting was successfully concluded and without a new mayor being elected or the mayor going crazy or being taken by a mood or whatever, the diplomat just suddenly went insane. It's not even winter yet, but he's swinging through the trees and running around babbling.
Oh, and a tree exploded at a fisherdwarf, who drowned in an ankle-deep murky pool.