This metal crossbow has a spherical glass reservoir of crimson blood at its base. It doesn't use regular crossbow bolts, instead it shoots squirts of infected blood at the target.
You have a +1 bonus to attack and damage rolls with this weapon, and its attacks deal necrotic damage. The crossbow also does not have the Loading property.
Blood Plague: Once per turn when you hit with an attack, you can attempt to infect the target with a virulent plague. The target takes additional necrotic damage equal to two rolls of its hit die. This happens automatically on a crit, even if you already used it on that turn.
Cursed: This weapon is cursed, and becoming attuned to it extends the curse to you. As long as you remain cursed, you are unwilling to part with the weapon. You also have disadvantage on attacks with other weapons.
Every time you use Blood Plague, you must make a Constitution saving throw (the DC starts at 1, and increases by 1 each time you use it).
Each time you fail the save, the curse takes deeper hold of you, but the DC is reset to 1. After the first time you fail, magical healing you take is halved, but your attacks with the crossbow deal a critical hit on a 19. After the second time you fail you no longer recover hit dice from resting, but recover one die every time you use Blood Plague. The third time you fail, the virulence fully takes over your body, and you become a bloodbeast under the DM's control.