I'll start by sharing an incident from one of my first fortresses. Having situated myself in a river delta with drawbridges going over the rivers as the only way into the fortress, a fell creature of the night laid an assault on my dwarves. A werechameleon had already ripped a few of my dwarves apart before someone had the sense to raise the drawbridges. Unfortunately, there was insufficient time for two dwarves to get inside before the bridges were raised, one of them being the mayor of the little hold.
With the way to the fortress cut off, the werebeast began targeting those that were still left outside. Not long after, only the mayor remained alive outside. In what proved to be a surprisingly fierce battle, the mayor and the werebeast tumbled into the river while trading blows in unarmed combat. Seeing this happen, the onlookers from the fortress started making a slab in memorial of the bravery displayed by the mayor, whose death was surely imminent. However, to everyone's surprise, the mayor climbed out of the water after being wounded and underwater an incredibly long while.
Although the hero was celebrated and reveled in their glory as the savior of the fortress, they also had to be initially quarantined due to fear of being infected by the wereplague. Although bitten multiple times, the hero seemed fine and was eventually free to roam the great halls again. That was, until new signs of darkness revealed themselves throughout the fortress. It started with dwarves laying about unconscious in their beds, refusing to work. Then some even seemed to fall asleep while working.
Not long after, there were found dead dwarves, whose bodies had been completely drained of blood. Something had to be done and a curfew was mandated with the population split into smaller groups in an attempt to find out what was going on and limit the spread of death. In one of the groups the dwarves kept dying and rumours started to spread about who had killed them. The populace began demanding justice be delivered. In a fury of rage, the dwarves of the smaller group were either killed by an unknown assailant or drowned in a chamber of execution.
At last there was only one dwarf left in the group of dwarves where the killer seemed to be: the mayor. Being the hero of the people, they had been spared the demands for retribution, and had been confined to their personal chambers for safety. Seeing they were the only one left, they could not be protected anymore and were sent to the execution chambers as well. However, just like the time with the werechameleon, the mayor seemed able to hold their breath quite well. In fact, they seemed not to breathe at all.
And that is the story of how I met my first vampire.