This item is found only in the console version(s) of Terraria.
The Vial of Blood is an item used to summon a Pet Bat. Vials of Blood are found in chests scattered around the world and in the dungeon. Like other Pet-spawning items, they have an infinite amount of uses. Using the Vial of Blood with the Pet Bat active will despawn it.
Like the Pet Zombie, the Pet Bat fights enemies alongside you. This is unlike two other pets, the Guinea Pig and the Pet Slime. You can also get this item on the latest update on mobile edition.