You have a +1 bonus to attack and damage rolls made with this magic weapon. In addition on a natural 20 all damage is converted to necrotic and the wielder is healed by as per the damage roll. If the wielder is at maximum hit points, the remainder is converted to temporary hit points. 

Cursed. Once attuned you do not want to give this weapon up and will resort to violence to keep it in your posession. An identify spell will not reveal the curse unless a DC 20 arcana check is rolled. To end the curse a remove curse spell must be cast while simultaneously applying the dying condition to a living target and soaking the crossbow in it's blood. The crossbow cannot be turned to another target until the curse is ended.

While cursed death saving throws automatically fail unless a natural 20 is rolled. If a natural 20 is rolled this only counts as a single success.

If a natural 1 is rolled to hit the crossbow hits the nearest ally to the target within 15 feet. Both the new target and the wielder take the damage and Strahd is healed by the combined total if the crossbow currently exists in Barovia.

Proficiency with a Heavy Crossbow allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, one willing to sacrifice their blood by expending 1 hp, Damage, Combat, Ammunition, Heavy, Loading, Range, Two-Handed, Push