Vampire
Urist McVictim, Cheesemaker has been found dead, completely drained of blood!
Vampires are undead night creatures that feed on blood, cursed during world generation by profaning against their gods. They occasionally appear in migrant waves and hide themselves amongst your dwarves. Vampirism can be further spread by drinking either vampire blood, or water contaminated by said vampire blood.
Description
Vampires, like other night creatures, are created during world generation. Every once in a while, a deity will curse a worshiper who desecrates their temple or otherwise offends them, cursing them to become either a vampire or werebeast. Any creature with blood, capable of learning, and not already a werebeast or undead, can, theoretically, become a vampire, but most vampires will be human or dwarven. However, since civilizations can have members not of their foundation race, the occasional vampiric goblin, elf or even animal person will also occur.
Vampires are much more powerful than normal humanoids, possessing enhanced speed, strength, stamina, and pain resistance in combat, don't need food, do not need to breathe (and thus cannot drown), and never get drowsy. They do, however, get thirsty, albeit not in the normal way; vampires thirst for warm, fresh blood, and will suck unconscious creatures (usually others of their own kind) dry given the chance, usually killing them. In the rare case that the victims survive and recover, they will not remember what happened to them, and may very well fall victim once more. It appears that when a vampire feeds successfully they receive a large happiness boost.
Vampires do not age, and most vampires live for hundreds or even thousands of years. Thus, all but the youngest vampires are more skilled and more experienced than their peers, and they are hiding their true identities. This makes them natural candidates for leadership, and thus vampiric monarchs are not an uncommon sight.
Vampires are a type of undead, therefore, animated dead creatures will be docile towards them, as will necromancers - while they're undead, necromancers cannot control vampires. Vampires can't have children.
None of your seven starting dwarves, children or babies will ever be vampires, nor most likely caravans, sieges, ambushes, or thieves, but any of the rest of your dwarves can be. Foreign diplomats can be vampires, and will be labeled as such.
Habits
Vampires are secretive and are a fairly common occurrence. Many fortresses can expect to see a vampire resident within the first few years, and some may see two or more. Vampires arrive with a false name and hide their true name and kill list until they are discovered. They act as do any other dwarves, performing jobs which are assigned to them and generally acting as expected, except for differences too small to notice easily in any sizable population: they do not eat, drink or sleep.
The most important difference is that sometimes, they drink the blood of dwarves that they catch sleeping. If any tame animals somehow fall asleep (for instance, via a syndrome), vampires will drink their blood as willingly as they will a dwarf's.
If vampires are caught in the act of draining a victim, their crime will be reported in the justice screen as murder. If only the corpse is discovered, the crime will be labeled as a murder sans suspects, and the player can accuse dwarves of the act. Even in the case that someone is accused, be aware that the deceitful vampire is capable of framing others for its crimes to send suspicion away for a time.
If a vampire is killed, the corpse will bear the original name of the creature rather than that of the dwarf who was seen to die. A coffin will be designated for burial of the vampire's cover identity, with the corpse bearing the original name entombed in it. Memorial slabs will be dedicated to the vampire's original name.
Uses
If you can correctly identify a vampire and isolate it from the rest of your population, you can make use of them without fear of blood feedings. A lone vampire in a sealed room will never die of hunger or thirst, does not need to sleep, and will never age.
Once you have your sealed vampire, your fortress becomes effectively eternal, since the vampire will always be alive. Consider placing a chair and table in your vampire's sealed room and making them an undead accountant. As they have nothing to do but sit around for eternity, once they get their skills up, they may make exceedingly effective managers/record keepers.
A cloistered vampire can also be used as a sleepless, un-eating and drinking dwarf who is always ready for some lever pulling. However, as vampires do not drink, yet are still alcohol-dependent, they will eventually suffer performance penalties and take longer breaks. This can have fatal consequences if you need the lever to keep the goblin siege outside pulled now.
Vampires do increase their stats like other dwarves, so that a weak vampire may be easily upgraded into a mighty one by using him as a miner or easily trained into a legendary swimmer. A vampire craftsdwarf may be burrow-limited to his workshop plus a stockpile or a miner restricted to specific mining levels, avoiding any other miners.
If you have no better idea you can use a vampire to explore the caverns; they are usually good fighters with military experience and will not run off to refill their waterskin.