The Corruption is a biome generated upon world creation and characterized by purple, decaying grass and trees. It has its own unique spawns and terrain, with the main features being giant chasms with Shadow Orbs and Demon Altars located at the bottom. Corruption has the unique ability to grow and spread, either naturally or artificially.

When starting out, Corruption spread is slow and outright stopped by certain blocks, most notably stone and sand. However, once Hardmode is unlocked, Corruption will spread faster and convert stone and sand into Ebonstone Blocks and Ebonstone Sand. The game will also generate a new type of Corruption biome called Underground Corruption that extends diagonally from the center of the Underworld to the surface.

In order for the achievement to pop, you must get at least 50% coverage. The Dryad NPC will track this for you, and sells Corrupt Seeds during a Blood Moon to help spread Corruption on the surface. However, Unholy Water, crafted from 1 Corrupt Seeds, 1 Bottled Water, and 1 Ebonsand Block, will be your primary tool in spreading Corruption.