Controlling the Corruption

This guide will help you slow down and prevent the spread of the Corruption/Crimson and Hallow in Hardmode.

Introduction

As stated in the guide description. this guide will help you stop the spread of the Corruption/Crimson, and Hallow. I will refer to these 3 biomes as the "world evil", or simply "evil". If there are any issues with the facts, spelling, or grammar, point them out in the comments and I will fix them.

What are these biomes?

The Corruption and Crimson are "evil" biomes present from world generation, and it is randomly picked which evil is generated, or after reaching a point in the game, you can choose it for new worlds. Note that the only way to have Corruption and Crimson in a world is to bring some of the alternate evil from another world. The Hallow is a new biome generated after beating the Wall of Flesh for the first time on any world, and will overwrite any existing biome when it generates.

The Corruption

The Corruption is identified by big, straight, chasms and purple vegetation. The chasms consist of several almost straight drops that can be deadly without fall damage preventing items, and a middle tunnel around halfway down. Under the middle tunnel in small caves containing a Shadow Orb, that when smashed with a hammer grants loot, a chance for a meteorite to fall at night, and when the third orb is smashed, the Eater of Worlds boss to spawn. Also, there are Demon Altars in the middle tunnel that can only be smashed in Hardmode, and when they are smashed, generate a new ore (1st, Cobalt or Palladium, 2nd, Mythril or Orichalcum, 3rd, Adamantite or Titanium) into your world and a chance a random block underground becomes Corruption or Hallow, boosting the spread.

The Crimson

The Crimson is similar to the Corruption, but with a bloody theme to it. It is identified with red vegetation and small hills of Crimstone that lead underground. The chasms follow a zigzag pattern before opening into a large main room, with more paths branching off to the Crimson Hearts, the Crimson equivalent of Shadow Orbs. Inside the main room and the entrance path, Crimson Altars can be found, which serve the same purpose ad Demon Altars. After destroying three Crimson Hearts, the Brain of Cthulu boss will spawn.

The Hallow

The Hallow is generated when a world enters Hardmode, in a diagonal area of the world going from the center of the world in the Underworld to the surface. The Hallow is identified by, red, yellow, green, and blue trees, and blue grass, and a rainbow in the background. Don't let the unicorns and rainbows fool you, the Hallow is a dangerous place. There are no special chasms, but new materials can be obtained from the new enemies on the surface and underground, like Souls of Light and rare drops such as the Rod of Discord, a teleportation item.

Slowing Down the Spread

Once in Hardmode, the evil will start to spread and if you don't take action, it will take over your entire world. Don't fear, there are ways to stop this spread!

Natural Barriers

Some structures naturally slow down the spread of the evil biomes. The Dungeon is the largest natural barrier, and if it stretches to the Underworld, completely seals off an area of the world from evil unless some spawns through a Demon/Crimson Altar. Corruption/Crimson and Hallow can not convert each other, so you can use one evil to block the other. The Snow Biome significantly slows down the spread, because the evil can not take over Snow, instead only Ice can be corrupted. The Jungle can not be corrupted by the Hallow, but Corruption and Crimson can turn Mud into Dirt and spread into the Jungle. Defeating Plantera will slow down the spread to 1/3 its previous speed. If there are 2 or more Sunflowers in a row, they can stop the evil from spreading, however the evil could spread below the Sunflowers through stone, making them ineffective as a primary barrier.

Player Barriers

The best way to slow it down is to make barriers in the world. The most common system is placing several hellevators across the world, although this is very time consuming. A normal hellevator may not be secure, though. It is a good idea to make it a few blocks wide, the evil can cross small gaps, so wider hellevators will do a better job. To make it faster, you can use Dynamite sold from the Demolitionist, or any Rocket Launcher with Rocket IV, sold by the Cyborg.
Corruption and Crimson grass grows thorny bushes, which can cross up to 6 block wide gaps, allowing it to spread across. If you line the hellevators with a non-corruptible block, like wood, and make sure there is some above the surface to block the bushes, you can easily stop the spread. An extra benefit of these barriers is it is easier to reach the underground areas of your world for resources, like ores.

Purifying the Evil

If containing the spread isn't enough, you can just get rid of it! There are two ways to do this. The first way is buying Purification Powder from the Dryad to cleanse the corruption, but this method is EXTREMELY SLOW considering the tiny range. The better option is using the Clentaminator item. The Clentaminator can be purchased from the Steampunker NPC for 2 Platinum Coins. The Steampunker will move in after at least one Mechanical Boss (The Destroyer, Skeletron Prime, or The Twins) has been defeated and there is a suitable house available. After buying the Clentaminator and a lot of Green Solution, go to the evil biome and start spraying it away.

Finding the evil to purify may be hard, but if you made hellevators to stop the spread, you can drop down them and spray the Clentaminator on your way down, or you can farm for the Rod of Discord and warp between caves while cleansing the world.

Conclusion

(decided to add it here instead of making a new section)
Thank you for reading this guide, I hope you found it helpful!
[EDIT] Thanks for all of the positive feedback!

[Irwin Rudolf]

o k a y t h x i m p r e t t y s u r e i c h e c k e d a l r e a d y b u t i l l c h e c k a g a i n

[Stamp20]

same place as the clentaminator

[Irwin Rudolf]

where do you buy green solution?

[A Random Dwarf]

but but but i like hallow it pretty

[jack black in real life]

World Evil / Hallow only spreads via foreground blocks so you don't need to worry about background walls.

[Yourname942]

Also, do I need to put playermade background walls between the wooden hellevator walls?

[Yourname942]

how wide should I make the hellevators? (how much space between the walls? Thank you.

[Stamp20]

stop it
get some help

[Ilikemozzarellasticks1234]

lol

[76561199001003825]

Thank you for doing this guide it has helped me a lot!