The Hallow

The Hallow is an end-game surface biome. It has an easily recognizable turquoise-colored grass, multicolored trees, and a huge rainbow in the background. It is the only biome where you'll find Unicorns, Pixies and Gastropods. The Underground Hallow biome can be found directly under a naturally-generated Hallow biome.

The most important property of the Hallow is that it does not exist in a newly-generated world. It only appears when entering hardmode, which is done by defeating the Wall of Flesh. Then, one (maybe more ?) Hallow biome along with a large Underground Hallow biome are generated. The Hallow can also be artificially created by planting Hallowed Seeds in dirt, using Holy Water, or placing Pearlstone or Pearlsand blocks.

The second important point about the Hallow is that it's the only environment through which the Corruption can't spread at all (the opposite also seems to be true). Consequently, its role is very important in hardmode, where the corruption becomes nearly unstoppable. The Dryad will tell the player the percentage of Hallow and Corruption in the world.

Although activating hardmode requires the player to have good equipment, the Hallow is a challenging biome, especially at night, since it's the home of powerful mobs. Don't be fooled by the cute and childish scenery.

Hardmode
As soon as Wall of Flesh is defeated, a section of Hallow spawns. This stripe goes from the Underworld to the surface, converting stone, sand and grass in its path into pearlstone, pearlsand and hallowed grass, respectively.

Hallow spreads in a manner very similar to corruption: There are also a few differences in the way that hallow spreads:
 * Hallowed grass can spread to dirt or grass blocks in the Surface Layer with at least one side exposed to air.
 * Hallowed grass can spread onto corrupt grass, and corrupt grass can likewise spread onto hallow grass. When the two meet, the results are unpredictable.
 * Stone can be converted to pearlstone.
 * Sand can be converted to pearlsand.
 * Pearlstone and pearlsand can spread through any depth, not just the Surface Layer.
 * Blocks that are up to three squares away from an existing hallowed block can be converted (even if unhallowable blocks are in the way).
 * Hallow can spread through its vines (but can not "go up" through regular vines).
 * Hallow cannot convert mud or jungle grass.
 * Hallow does not uproot sunflowers (but can still spread beneath them).
 * Pearlstone does not seem to create hallowed grass, or at least not as readily as ebonstone creates corrupt grass.

Pre-hardmode
Hallow does not normally exist before the world enters hardmode, but can be created with Hallowed Seeds or Holy Water imported from another world. If it is created, it will only spread the same way as pre-hardmode Corruption: slowly, on the surface, and only through dirt and grass. It still prevents the Corruption from spreading. Furthermore, Hallow-specific mobs won't appear, since they were only designed for hardmode.

Hallowed Desert


When the Hallow spreads through desert, it becomes Hallowed Desert. It is very similar to the Hallow: the music theme is the same, the mobs are the same (except the Light Mummy), and the background is slightly different.

The sand is replaced by pearlsand. Although their color is different, cacti still drop cactus when cut, and waterleaves are also able to grow.

History

 * Early mentions: Terraria Screenshot on Steam 1 28th November 2011