Hallowed Prism Wall

Mining the wall out of an Underground Hallow returns nothing at all, which is standard for wall patterns the world generates for itself. Crafting is the whole supply chain here.

Every hallowed wall pattern is a rewritten cave wall. Prism is the Hallow's version of the Worn Stone Wall; Mottled becomes Shard, Fractured becomes Crystalline, and Stalactite becomes Cavern. Hallow spread does this for free as it moves through a cavern, and undoing it takes a Clentaminator firing Green Solution. Nothing else reverses it.

Wall that came with the world does not block spawns. Wall you place by hand does, and the two are visually the same, so a hallowed base looks correct and still keeps enemies out as long as you replaced the background rather than leaving it as you found it.