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.