Upon the activation of Hardmode, Hallow and Corruption/Crimson start spreading, potentially converting the entire world. This guide aims at providing strategies about intervening in this and maintaining world purity.
Since the biomes spread at a relatively slow speed, especially after defeating Plantera and in pre-Hardmode, it takes quite a long time for them to completely convert even a small world. As a result, containing the biomes needn't be considered urgent, especially for shorter playthroughs. However, some worlds are played on for many hours, and if not dealt with, the Hallow and evil biomes will convert most or even all of the world, and add many difficulties for the player.
Whilst Corruption and Crimson spread slowly and only via grass and thorny bushes during pre-Hardmode, they spread very aggressively and to a multitude of different blocks once the Wall of Flesh has been defeated. Defeating Plantera slows the overall spread speed by 50%.
Although near identical in spread there are a few key differences between the three, being the Corruption's inability to spread by foliage, crimson grass's inability to resist conversion from hallowed grass, and the Hallow's inability to spread through the jungle, the last of which is particularly notable for preventing permanent Jungle destruction.
Prior to entering Hardmode, it is recommended to contain the existing areas of the Evil Biome.
The simplest step, and often the first, is to dig a tunnel 3-6 blocks wide completely surrounding the areas. Another critical step is placing non-corruptible blocks (such as wood, snow, hay, dungeon bricks, etc.) over or replacing any exposed top sides of corrupt/crimson/jungle grass within 6 blocks of the tunnel's edges and along the floor of the tunnel, as well as any exposed bottom sides of any grass type along the ceilings of your tunnels to prevent thorns & vines. (Even if Corruption doesn't have vines of its own, it can travel through normal or jungle vines.) If you have not or chosen not to cover any top side grass that may grow thorns, make sure to at least leave 5-7 blocks sticking out from the tunnel's sides to prevent spread from thorns that may grow over the tunnel. It is also a good idea to build these tunnels around areas of interest, such as the Jungle or the player's base.
An effective way of digging these tunnels quickly due to the minimum 3-block width is using the Scarab Bomb. The horizontal and vertical explosions leave tunnels of exactly 3-block width, allowing you to chain multiple together for quick and efficient tunnels. However, the width of diagonal explosions leaves some blocks within the 3 block-reach of Hardmode spread, therefore it is recommended you only use the direct horizontal and vertical explosions.
Sunflowers are a viable option for early containment, as they can limit surface spread of both evil biomes, even in Hardmode. However, they become less effective when the evil biomes begin to spread underground in Hardmode, so use them mostly for holding back surface grass spread while building your quarantine tunnels in Pre-Hardmode.
While purifying your world evils with purification powder can be tempting, it is almost always better to tunnel around the biome and wait to unlock the clentaminator, as even in Pre-Hardmode total world purification with powder is extremely costly and unlikely to succeed unless the player is very thorough.
Shortly after the Wall of Flesh is defeated, two diagonal strips of Hallow and Corruption/Crimson will generate in a V shape. The location of these strips and distance between them is completely random, but if the player has done a lot of previous underground exploring, the affected areas will be very clear. These diagonal strips will begin spreading as soon as they appear, so they can wreak havoc if left unattended. However, immediately trying to stop the spread is not recommended. The new enemies can easily kill players at this point, and the tools available are not powerful enough for the task. Therefore, it is wiser to wait until better gear is acquired.
In a long term world the player's first priority should be preventing the spread of world evils into the Jungle, as Jungle reconstruction is extremely tedious and must be done block by block. While not many options are available before unlocking the Steampunker, if a safe zone underground is set up the player may be able to slow the spread by planting Hallowed Seeds (bought from the Dryad for 20 silver) or pearlstone at the border. This is especially effective against the Crimson, as crimson grass in converted Jungles is quickly overtaken by hallow grass.
While biome spread is constant in all block types the amount and concentration of convertible blocks greatly affects the speed of spread, with spread being fastest in Beaches and Deserts, followed by Forest, Underground Snow, and Jungle, with convertible ice blocks being too spread out within most of the surface Snow biome for spread to be possible, outside of World/Hardmode generation and player intervention. Mushroom grass is also immune to spread, although the world evil can still turn nearby mud blocks to dirt.
As blocks are updated 6 times more often on the surface, biome spread is affected accordingly, with the Corruption, Crimson, and Hallow spreading 6 times more quickly there.
In order to obtain this gear faster, one might consider smashing numerous Altars in order to find the new Hardmode ores more quickly. However, note that every single Altar smashed can turn a random block (that can spawn anywhere) into its Hallow/Evil counterpart. This will barely speed up the overall spread, but can be very harmful if the blocks spawn outside quarantined areas. A minimum of 3 altars must be broken in order to gain access to all Hardmode ores, but they can technically be skipped if the player is able to defeat a mechanical boss without their help.
If the world evil spawned near the Jungle, it can be helpful to artificially create a perimeter of Hallowed blocks to prevent the Evil Biome from converting the jungle, as the Hallow will not spread to the jungle. In any case, note that chlorophyte ore (which can convert dirt and normal/converted grass backs back to mud and jungle grass) does offer the Jungle some protection against the evil biomes.
Once a Mechanical Boss has been defeated, the Steampunker will arrive, giving the player access to the Clentaminator, which helps massively in cleaning and preserving world biomes. Note that this tool and its Solutions are fairly expensive, so the player should have the necessary funds at their disposal; see Guide:Making money for further reading.
- Green Solution is helpful in purifying vast portions of the world though not always the best option, as purified blocks can and will be re-converted unless the purified area is quarantined or the world evils and Hallow are eradicated first.
- Blue Solution can be useful in hallowing portions of the world evil near or adjacent to the Jungle, but caution is advised as Blue Solution itself can convert jungles into Hallow despite not spreading to Jungles normally.
- This can be worked around by using Dark Blue Solution on nearby Jungle terrain first, since light Blue Solution cannot Hallow mushroom biomes.
- Dark Blue Solution can be used as a cheaper and less infectious replacement for Light Blue Solution in preventing Jungle conversion, as Mushroom Grass is immune to all forms of conversion thereby protecting it, and can be changed back to Jungle later with Green Solution.
- Blue Solution, Purple Solution, or Red Solution can also be useful in conjunction with hellevators in order to create contained pockets of a particular spreading biome. Hellevators make it easy to clean up the biome after its purpose has been served, on top of allowing you to easily travel to any height level within.
- Buffs that give a chance to not consume ammo affect the Solutions from the Clentaminator. With enough of them, a lot of money can be saved as the player can spray with smaller amounts for much longer.
If you want to make cleansing your entire world a feasible task, the Clentaminator is an essential tool.
The most reliable way to completely convert or purify a world is to dig multiple Hellevators with a horizontal distance of 120 blocks (since 60 tiles is the maximum range of the Clentaminator) between each other, then glide down each one using Wings or a Featherfall Potion, while spraying both sides. Additionally, the Clentaminator produces a decent amount of light that can completely go through blocks, mapping out the entire world. When purifying it may be advisable to equip a Shield which grants knockback immunity, as taking knockback from enemies while falling down a hellevator can be very irritating.
As desirable as completely eliminating the spread might sound, leaving a part of the world with Corruption/Crimson and Hallow is not a bad idea if contained properly, since there are a lot of items exclusive to these biomes the player may need later, such as Souls and Biome mimics.
- Thorns can grow from Crimson, Corrupt, or Jungle grass. Corrupt and Crimson variants can grow 6 tiles in all directions, and spread their biome up to three blocks away like any other source, but their spread can be contained with physical neutral barriers, like Clay or Wood. Jungle thorns can spread into range of Corruption, become corrupted, and spread further into the Jungle. If left unchecked, thorns can inadvertently spread the Evil across otherwise safe gaps.
- The Dungeon can completely prevent the spread of Hallow and Corruption/Crimson if it reaches down to the Underworld. If it doesn't reach that far, it can at least slow it down.
- Floating Islands can be turned Hallowed or Corrupt/Crimson during the initial generation from Hardmode, or if converted blocks are placed near them.
- It is much easier to maintain world purity in not the bees secret world seed, because on surface you can create artifical mushroom biome, near evil biome, and clorophyte will protect cavern layer. Hallow is almost no threat at all because almost entire world is either jungle, or hive. The only danger is desert, because its hard to maintain desert purity.