This is like my 3rd fort. I'd gotten things up and running quite nicely, was just about to start getting my military preps going after about a year of buying iron, steel, etc. (I find that getting armour together is very hard and time consuming). Brought a load of coal with me on embark too, so I have tons of that at least.
Was digging down about 30 levels looking for some metal ore and breached a cavern. Was close to walling it up when I see a giant bat making a beeline for my staircase. Issue a civilian alert to get to the safe zone, which is the bedroom area. Everyone gets to the bedroom area, but that's just based around the stairwell. Giant bat zips up the stairwell, then up into the kitchen area and starts attacking a metalworker dwarf. Metalworker dwarf is not in a good state.
Drafted an emergency squad containing my 2 miners and woodcutter (at least there are picks and an axe in there...) and sent them to kill the bat. By the time they reached it, the metalworker dwarf was dead, and the bat had taken a few bruises. My squad went for the bat. Quite a bit of blood, but rather shortly my heroic woodcutter smashed his axe through the bat's skull! Several dwarves with broken bones and dripping blood, next priority is to build a hospital, stat.
The cavern stairwell is now sealed up with a stone floor. To add insult to injury, I just had a thief stab and injure one of my fisherdwarves too. Think I'm going to batten down the hatches, close up the front drawbridge, get a hospital built, and then try to clean up the blood.