Look to your world's Cavern layer for an area that is neither Corruption/Crimson nor Hallow, preferably with some nice tunnels already there so that it's easier to carve it out. Use your map or explore to find a suitable location. While I'm making an underground Corruption Biome in this tutorial, the process is the same for Crimson.

All varieties will spawn, including mimics which can drop unique accessories and weapons. If you're going through the trouble of doing this, you probably want the best material and gear drops available, and those typically come from underground monsters. If you have a Crimson World and want Worm Teeth to make Thorns Potions, you will need a surface Corruption, as the World Eater spawns instead of the Devourer in the Underground Corruption. The rock you place will transform a layer of dirt above into the evil grass, resulting in mushrooms growing which are mainly used to summon the two bosses unique to Corruption and Crimson (Eater of Worlds and Brain of Cthulhu). If you want grass to spawn, layer corrupted rock then dirt on top. You can, of course, do it directly on the surface, but it would be wise to carve around it so that it cannot spread - at least 4 spaces to prevent any mistakes. Point is - stone and dirt do convert, and you only need one corrupted rock to do it but more results in faster conversion.ДЄll you need to do to get an above-ground one going quickly is build it in the sky - not so far Harpies will spawn, but far enough that it can't spread and ground monsters won't spawn either. Some types of tiles can't be converted, but that's outside the scope of this tutorial. They will jump up to 3 tiles away and convert stone to ebonstone/crimstone and their special grass will take over dirt. Once you enter Hard mode, Corruption and Crimson spread rather fast, but not so fast you should worry. Just get Crimstone/Ebonstone and save/exit. There is no need to wait on a Blood Moon and buy the seeds from the Dryad. If you simply have those, you're good to go on making your own Biome. Mine the Crimstone/Ebonstone blocks of the area once you find one, then take them to your main world by leaving the game with them in your inventory.

Hope it's the opposite kind you have in your main world, and if not make another and delete that one unless you like it. To get started, simply head off to Terraria's main menu and generate another world, preferably a small one, to investigate with your character. It is best done after the start of Hard Mode, because of the stripes of Hallow/Corruption that will spread when you initiate it. These are craftable after beating Brain of Cthulu or Eater of Worlds. While it takes many more blocks for these 'normal' Biomes to change their background and spawns, Corruption and Crimson only require a few hundred blocks! That's easy to attain, so long as you have at least a Deathbringer or Nightmare pickaxe. Some biomes are compatible with one another - for example you can have Cavern enemies spawn in an Underground Crimson Biome, Snow enemies in a Corruption. Once a certain block threshold is met, monsters from that biome will start spawning from those types of blocks, and when you're nearby you can see the background has changed. Biomes are basically big collections of blocks within a given area, and have their own background, music, and monsters. The type of monsters that spawn are determined by the Biome those blocks are considered to occupy. Terraria handles spawning off-screen, based on the block types outside of your view. How Terraria Handles Spawns & Determines Biomes This is entirely doable! This tutorial will teach you how Terraria decides where to spawn monsters, what type of biome it is, and how you can get a Corruption or Crimson (mainly underground varieties) going in your world.

So you've got a Corruption or Crimson World and want to have the other Biome present for farming materials and/or rare drops. Terraria Tutorial: Making Corruption, Crimson, or any other Biome A Corruption Biome that I made in my Crimson World, just to test how it all worked.

Terraria making a crimson desert biome