Corrupt Grass Wall

The version that generates in a corrupt biome spreads the corruption to walls near it. A copy you place yourself is inert. That distinction is the whole reason buying them matters: you can build a corruption themed room inside your base without watching it eat the rest of the house.

You cannot mine one out of the ground, so the Dryad on a blood moon is the only source and bulk quantities add up fast. When you want to corrupt walls that are already up, Unholy Water is the precise tool. The Clentaminator covers a wide enough column that it will convert blocks and other wall types you never intended to touch, which is difficult to undo cleanly.

Corrupt these from flower or jungle walls and the little flowers carry over into the corrupt version. Cleanse them back and the flowers survive that too, even though a purified wall normally comes out plain.