I was around 7 hours into my first fortress. I had more or less figured out how to avoid running out of food and drink, so everything was more or less going smoothly. So thought the game apparently, who figured everything was going too smoothly for its liking and decided to bring a new friend to my fortress. This new friend was a big quadruped monster made out of amber and who also spat some kind of acid on everything in its path.
Now, you might be thinking "Well, just send a few soldiers, they'll take care of it surely," and that would be a very sound and reasonable idea, were it not for the fact that there was absolutely no military of any kind in my entire fortress.
You see, dear reader, up to that point I had not encountered anything remotely similar to an enemy (with the exception of goblins trying to steal logs from time to time), so I didn't really want to spend resources and time into training a squad or two.
I had to come up with some kind of solution of sorts to this predicament. That beast was out there and it would eventually reach my dwarves, I had to do something... And so did I.

I closed the doors. I know, I know. It doesn't seem like the most effective solution, but I didn't have many other options at the ready and the vast majority of my dwarves were already inside, so it was good enough.

Back to the beast, it started moving towards the few dwarves that were left outside, each one of them far away from the others. It killed the first one without even breaking a sweat and then went for the second one, who did put up more of a fight, dodging about five blows before finally getting grabbed by one leg and thrown against a rock, dying instantly. The third and final one couldn't do anything, much like the first one.
The beast was now on its way to my humble fortress. I was, of course, trying to come up with a plan in case it got in, because I certainly didn't think the doors would hold up for long, but as it got closer and closer, the beast decided to turn left, towards my animals, to slaughter them all. They tried to fight back too, but in the end the beast won with just a few scratches.
Fortunately, the monster had moved far away from my fortress while it was chasing one of my animals. This was good, as I had ordered my dwarves to start making weapons and armor so that we could fight back and this would buy us some precious time.
In the meantime, I noticed an old man coming from the southwest to visit us. Unfortunately for him, the beast happened to have recently killed one of my animals in that place and was still hanging around. When the old man saw the beast he immediately fled away as fast as he possibly could. The beast either didn't see him or didn't care enough about him because it didn't chase him.
A few moments later, while I was still watching our dear monster, I saw a zombie approaching the beast from the west. Contrary to what I was absolutely sure was going to happen, this new creature found some way to not only fight and hurt the beast, but to even kill it.

Now, I was confused. Where did this zombie come from? Why did it fight the beast?

After digging for a bit, I found out that the zombie was none other than that dwarf who was able to somewhat keep up with the beast when it first appeared. But why was he a zombie? Well, that was the old man's doing. He was a necromancer. Apparently he stumbled upon the dwarf's dead body and decided to resurrect it.

And so, my fortress was freed after this necromancer visitor had come, by sheer luck, to rescue it from that beast.

He was, unbeknownst to him most surely, a hero.

Everything went back to normal, except for five dwarves who had now to start training to become soldiers. The necromancer had decided to stay in my fortress as a guest. I hoped he would eventually want to become part of my fortress as a citizen.

Or that's what I wanted until, a few months later, my captain of the guard found he was also a spy, to which she responded by beating him almost to death and chaining him up for a few years.