Sanguinis Luxuria is a mod about becoming a vampire.
When killed, villagers can leave some blood behind on the ground. This blood can be picked up with a bottle, and drank.
Drinking blood gives very little to no hunger, and gives nausea and blood sickness, among other less than desirable effects. Every time you drink blood, the level of blood sickness increases by one. Drink too much, and something interesting may happen...
Vampires require blood to survive. Luckily, as a vampire you are no longer dependent on blood left behind by villagers. Instead, while looking at an entity with blood, you can hold down the bite key (r by default) to slowly drain their blood. Do be careful, however, as once you completely drain something's blood, it will die.
As a vampire villagers will also be scared of you, and Iron Golems will attack you on sight. Fortunately, there is a solution. By wearing a Carved Mask villagers won't run away, and Iron Golems will no longer try to kill you on sight. It does come at the cost of being unable to drain blood, however.