The Hallow

The Hallow is an "anti-Corruption" biome which is unlocked by defeating the Wall of Flesh. The Hallow can be created by planting Hallowed Seeds or throwing Holy Water and will stop the Corruption biome from spreading. The biome has light blue grass and colored trees. It also has its own underground biome which is where Pearlstone is located.

The Dryad will tell the player the percentage of Hallow and Corruption in the world, after the player has defeated the Wall of Flesh boss in the Underworld.

Prevention
Mud or Clay can stop The Hallow from spreading if its placed correctly with no grass growing around the blocks.