Holy Water

Holy Water is a consumable weapon which transforms a small area into The Hallow when thrown. It can be used as a weapon, but it deals comparatively little damage at the stage in the game when it becomes available. It is recommended over the Hallowed Seeds for spreading Hallow due to its ability to spread the Hallow to all applicable blocks in its AoE and the fact that you only need to buy 1 Hallowed Seed for each stack of 5 crafted.

There is also Unholy Water, which has an opposite effect.