Terraria

The Hallow is the opposite of the Corruption, generated upon unlocking Hardmode, and features bright and colorful trees and enemies. It spawns diagonally from the center of the Underworld to the surface once the Wall of Flesh is killed. The underground portion of the Hallow is called the Underground Hallow, which mirrors the Underground Corruption in a V-shape. Much like Corruption, it will spread through stone and sand and convert them into Pearlstone Blocks and Pearlsand Blocks.

In order for the achievement to pop, you must get at least 50% coverage. The Dryad NPC will track this for you, and sells Hallowed Seeds once the Wall of Flesh is killed. Using those seeds to make Holy Water is better for spreading the Hallow than planting them. Holy Water requires 5 Pixie Dust, 1 Bottled Water, and 1 Hallowed Seeds.

There are 3 alternative ways to get the achievements. They are listed below from easiest to hardest.

Method 1:
Buy a Clentaminator from the Steampunker NPC. The Clentaminator is a tool that uses different color solutions to change the landscape it is fired at into a different biome. It is rather expensive, ringing up at 2 platinum coins, but it will make getting both this achievement and the "Corruptible" achievement significantly easier. Also, make sure to buy plenty of Blue Solution (which is sold by the Steampunker NPC when she is in a Hallow Biome), as this is the ammo used by the Clentaminator to spread the Hallow.

Once this is done. Exit out of your world and start up a new world. Make sure that this world's size is set to small, as this will make it go the quickest. Once in the world, simply fly around while shooting all the terrain with the Clentaminator. This will change all of the land to the Hallow. Once you reach 51% Hallow, the achievement should pop. It might make it easier to quickly defeat one of the bosses and build a house to get the Dryad NPC, as she can tell you how much of your world is Hallowed.

Method 2:
Another way to get the achievement is to craft a large amount of Holy Water, which is crafted using 5 Pixie Dust, 1 Bottled Water, and 1 Hallowed Seed. Once this is done, load up a small sized world, and fly around throwing the Holy Waters on every bit of ground. This will transform the ground into Hallow. Again, it may be helpful to kill one boss and build a house to get the Dryad NPC, in order to know how much of your world is hallowed. If you can not get to 51%, kill the Wall of Flesh to put the world into Hardmode. In Hardmode, the Hallow spreads significantly faster.

Once in hardmode, go back to the surface and make sure to change the recently spawned Corruption biome back into Hallow. After this is done, simply AFK for some time to allow the Hallow to spread. The achievement should pop once you reach 51% Hallow.

Method 3:
This methods is probably the most time consuming of the three, however it involves not crafting of items, and only a relatively small amount of mining. Hallowed blocks, such as Pearlsand and Pearlstone can spread Hallow to blocks near it, although rather slowly.

First, go and mine a bunch of Hallowed blocks, at least one stack. Once this is done, load up a new small world. Immediately go down to the underworld and defeat the Wall of Flesh. Once this is done, go back up to the surface and surround the Corruption in Hallowed blocks, this will hopefully spread the Hallow to the blocks surrounding the Corruption and stop it from spreading. Then take your remaining blocks and spread them all over the world. Once this is done, you will have to wait quite awhile for the Hallow to spread enough to get the achievement.

The Hallow is the opposite of the Corruption, generated upon unlocking Hardmode, and features bright and colorful trees and enemies. It spawns diagonally from the center of the Underworld to the surface once the Wall of Flesh is killed. The underground portion of the Hallow is called the Underground Hallow, which mirrors the Underground Corruption in a V-shape. Much like Corruption, it will spread through stone and sand and convert them into Pearlstone Blocks and Pearlsand Blocks.

In order for the achievement to unlock, you must get at least 50% coverage. The Dryad NPC will track this for you, and sells Hallowed Seeds once the Wall of Flesh is killed. Using those seeds to make Holy Water is better for spreading the Hallow than planting them. Holy Water requires 5 Pixie Dust, 1 Bottled Water, and 1 Hallowed Seeds.

There are 3 alternative ways to get the achievements. They are listed below from easiest to hardest.

Method 1:
Buy a Clentaminator from the Steampunker NPC. The Clentaminator is a tool that uses different color solutions to change the landscape it is fired at into a different biome. It is rather expensive, ringing up at 2 platinum coins, but it will make getting both this achievement and the "Corruptible" achievement significantly easier. Also, make sure to buy plenty of Blue Solution (which is sold by the Steampunker NPC when she is in a Hallow Biome), as this is the ammo used by the Clentaminator to spread the Hallow.

Once this is done, exit out of your world and start up a new world. Make sure that this world's size is set to small, as this will make it go the quickest. Once in the world, simply fly around while shooting all the terrain with the Clentaminator. This will change all of the land to the Hallow. Once you reach 51% Hallow, the achievement should unlock. It might make it easier to quickly defeat one of the bosses and build a house to get the Dryad NPC, as she can tell you how much of your world is Hallowed.

Method 2:
Another way to get the achievement is to craft a large amount of Holy Water, which is crafted using 5 Pixie Dust, 1 Bottled Water, and 1 Hallowed Seed. Once this is done, load up a small sized world, and fly around throwing the Holy Waters on every bit of ground. This will transform the ground into Hallow. Again, it may be helpful to kill one boss and build a house to get the Dryad NPC, in order to know how much of your world is hallowed. If you can not get to 51%, kill the Wall of Flesh to put the world into Hardmode. In Hardmode, the Hallow spreads significantly faster.

Once in hardmode, go back to the surface and make sure to change the recently spawned Corruption biome back into Hallow. After this is done, simply AFK for some time to allow the Hallow to spread. The achievement should unlock once you reach 51% Hallow.

Method 3:
This methods is probably the most time consuming of the three, however it involves not crafting of items, and only a relatively small amount of mining. Hallowed blocks, such as Pearlsand and Pearlstone can spread Hallow to blocks near it, although rather slowly.

First, go and mine a bunch of Hallowed blocks, at least one stack. Once this is done, load up a new small world. Immediately go down to the underworld and defeat the Wall of Flesh. Once this is done, go back up to the surface and surround the Corruption in Hallowed blocks, this will hopefully spread the Hallow to the blocks surrounding the Corruption and stop it from spreading. Then take your remaining blocks and spread them all over the world. Once this is done, you will have to wait quite awhile for the Hallow to spread enough to get the achievement.