The HallowIt is an anti-Corruption biome and can be spread by planting Hallowed Seeds or throwing Holy Water and is most effective when used to counter the spreading of Corruption. However the corruption and hallow can spread over each other still, just very slowly.
The Hallow can be spread above ground with Pearlstone Blocks, Pearlsand, Holy Water, or with Hallowed Seeds.
The rules for spreading The Hallow are as follows:
Pearlstone and Pearlsand Blocks will spread The Hallow to adjacent Stone, Sand, and Grass blocks (Pearlstone will not spread The Hallow to dirt blocks without Grass growing on them). When planted, Hallowed Seeds will spread The Hallow to adjacent Dirt, Stone, Sand, and Grass blocks.
The Hallow cannot spread to Mud Blocks or snow blocks. Hallow will not spread through gaps in the environment that are three or more blocks across.
Hallow will not spread through synthetic blocks (i.e. materials such as Gray Brick)
Vines growing from hallowed ground can spread The Hallow through proximity.
The Hallow can be planted in a world where the Wall of Flesh has not been killed but it won't spread through Sand and Stone. It will spread the hallow grass over default grass, however.
If you want to get rid of the Corruption (after beating the Wall of Flesh of course) an easy way would be to spread the hallow there with hallow seeds, some holy water or by placing different Hallow blocks. You can use Holy Water in a world where there is no hallow, and it spawns it, though it is slower.
Items neededHoly Water
Tooltip Spreads the Hallow to some blocks
Crafting- Makes 55
Crafted With
Pixie Dust
1 Bottled Water
1 Hallowed Seeds
Holy Water is a thrown item that transforms the blocks it hits into Hallow. A more practical use is removing Corruption. Holy water is an effective method of removing the Corruption, arguably more so than Purification Powder because the latter reverts corruption to neutral stone/grass/sand, which will shortly become corrupt again, especially when playing in Hard Mode.
Holy water on the other hand Hallows whatever it touches, meaning these areas will stay cleansed for a much longer period of time.
Hallowed Seeds
Information
Purchase From Dryad
Purchase During After defeating Wall of Flesh
Material In Holy Water
Hallowed Seeds are seeds that spread the Hallow. Hallowed Seeds can be placed on Dirt Blocks. They can also be brought onto worlds in which hardmode has not been activated, and the hallow can be spread there as well. Because of their price tag and the fact that the Hallow can also be spread by placing Pearlstone or Pearlsand, they are best used to craft Holy Water.
Hallowed Seed is the only seed ingame which has the Golden tier (Tier 3), whilst all other seeds are only tier 1.
Pearlstone Block
Information
Found In Underground Hallow
Material In Pearlstone Brick
The Pearlstone Block is a Stone Block that has been claimed by The Hallow, and will expand The Hallow until another biome (such as the Jungle) interrupts the spread.
Pearlsand Blocks cannot be used in place of regular Sand Blocks to smelt Glass. However, it can be placed and reverted to regular sand with Vile Powder.
Pearlsand Blocks can be used as ammo in the Sandgun.
Information taken from
http://terraria.wikia.com/wiki/Terraria_Wiki