Hardmode conversion

The first Wall of Flesh kill in a world triggers a single terrain rewrite that never repeats. Two diagonal stripes are cut through the map in a rough V, each running from the top of the world down to the floor of the Underworld. One converts everything it crosses to the Hallow, the other to whichever evil biome your world has. The Hallow stripe is always drawn first. When the line about ancient spirits being released shows up in chat, the pass has finished.

Where the stripes land is decided by world size and by where the Dungeon sits. It has nothing to do with where you were standing when the boss died. Dragging the fight to the far end of the Underworld to keep the conversion away from your base does not work, and it is the most repeated bad advice about the transition. Which arm gets Hallow and which gets evil is a coin flip, and the two can land far apart or overlap near the middle.

The stripe cuts through whatever is in its path and the one place that genuinely hurts is the Jungle. Jungle grass converts, and a converted Jungle stops producing Life Fruit and Chlorophyte Ore, both of which sit on the critical path through Hardmode.

Dig a vertical trench down each side of the Jungle from the surface to the bottom of the cavern layer before you take the fight. Spread can cross a short gap, so make it at least three tiles wide and give it more if you want margin. Remember that vines and thorny bushes reach across a narrow trench and plant grass on the far side, which is how most quarantines actually fail. Lining the trench with a block the spread cannot claim, plain wood or any crafted brick, holds better than empty air. The same treatment around a base is worth doing, though a base is far easier to clean up later than a Jungle is.

Purification Powder from the Dryad works block by block and is miserable at this scale. The tool that actually solves it is the Clentaminator, which the Steampunker sells once a mechanical boss is down, together with the Green Solution that reverts ground to normal. Until then containment is cheaper than cleanup, which is why the effort belongs in the window before you ever hit the Wall.

After the stripes are drawn the game runs a second, quieter pass over background walls. It picks a random tile between 100 tiles above the surface and 190 tiles above the world floor, checks the block against a list, then measures the connected pocket of open space above it. If that pocket holds between 50 and 1000 tiles and its existing walls are all replaceable, the whole pocket is repainted with one biome wall type.

This is why a cave you already cleared can suddenly register as Hallow or Corruption while the blocks in it look untouched. The number of successful replacements is capped per world size, and the game also burns an attempt after a long enough run of failures, so the coverage is deliberately patchy rather than complete.

Drunk worlds contain both evils, and the stripe converts to whichever of the two is dominant on the day you kill the Wall. Add For the Worthy on top and the stripe carries both at once. Remix and Zenith throw out the V shape entirely and use a rectangular conversion zone on the same side as the Jungle, and it only creates Hallow. No new evil biome is generated in those worlds at all. The full list of what each one alters is under secret seeds.

The conversion does not place Cobalt, Mythril or Adamantite. That happens separately, when you break Demon or Crimson Altars with the Pwnhammer the Wall of Flesh drops. Each break seeds a batch of ore somewhere in the world and also has a chance to scatter a patch of evil biome, which is worth knowing before you smash altars inside the area you just spent an evening trenching.