Realms

Realms are a feature for NTerraria that allows players to "teleport" to another realm, in which blocks and enemies are changed.

The NPC's (except the realms bunny and the guide, includes enemies) and the dropped items won't teleport to another realm, but stay in the same realm. Lighting, however, remains the same for all the realms.

The way it works is that it takes all blocks from the normal realm, and transform it to the new realm's counterpart of that block. For example, If we go from the Normal Realm to the Bloody Realm, the (pure) grass will become crimson grass.

There are 11 realms:

(Disclaimer: Becomes means that it switches blocks with a valid counterpart to it's counterpart for example, Crimson becomes Hallow means all the crimson-related blocks will become hallowed-related blocks) The realms are the only way to get corrupt/bloody items from a bloody/corrupt world without making a new one therefore making the mod perfect for 100% completion and challenge.
 * 1) Normal Realm: Everything you know and love about your world. Basically normality.
 * 2) Evil Realm: Everything becomes corrupted. The exception is the Jungle, which for world safety and stability reasons, becomes Mushroom.
 * 3) Bloody Realm: Everything becomes crimson(ed). The exception is the Jungle, which for world safety and stability reasons, becomes Mushroom.
 * 4) Evil Division Realm: The world becomes split in half (the natural spawn coordinate is the division point). This way, one side will become completely corrupt, and the other side will become completely crimson. The exception is the Jungle, which for world safety and stability reasons, becomes Mushroom.
 * 5) Evil Chess Realm: The world becomes a check board: The white will be crimson and the black will be corruption. This one is buggy and glitchy. The exception is the Jungle, which for world safety and stability reasons, becomes Mushroom.
 * 6) Hallowed Realm: The world becomes hallowed. The Jungle stays the same in this realm because it has no risk of being swallowed by the hallow.
 * 7) Napolitan Realm: The world divides in 3: Corruption, Crimson and Hallow. If the jungle falls into Corruption/Crimson domain, it will become mushroom. Else will stay the same.
 * 8) Spider Realm: All the empty spaces existent will become Spider nest blocks, making it able to spawn Spiders. The kind of spider will change depending whether the world is hardmode or not. Intentionally, you cannot place torches in this realm for two reasons: If the world is dark, it will be spookier and more fun to fight spiders and the players won't be able to place torches in walls, therefore when returning, become flying torches.
 * 9) Night Realm: The sun stops providing light. But it's still day, meaning that you cannot spawn bosses during the dark day.
 * 10) Weird Realm: Everything becomes odd. But to avoid spoiling player's fun by telling what there is, will stay untold in the Wiki unless told the opposite. The mobs will have randomized AI, For example, a bunch of Slimes with the AI of the golem, or a zombie with the AI of the Dungeon Guardian (which is actually pretty fun)
 * 11) Reverse Realm: The only realm that isn't linked to the Realms Bunny. To reach this realm you will need to kill the Duke Fishron (most powerful boss of the mod), which will always drop the reverse mirror. Look at it and you will be teleported the Reverse Realm. It's your world, just reverse, in which all mobs have their normal levels, with a little tweak: ADD 100 TO THAT NUMBER. That realm is the one in which you can spawn the Vengefull Eye of Cthulhu and the Zone Eater.

Raccoon Trivia
This was my attempt of transporting you to different versions of the world, while being in the same world.

The Realms system basically masked the tiles in the world for a tile of a different type, and also only showed and allowed interaction with npcs from the realm you were at. But on the side note, that initially caused some problems with world saving, because I had forgotten to make the game world save the original version of the tiles. Without counting that the corruption and crimson could also affect the real world, causing some tiles to appear corrupted/crimsoned once you get back from the realm that had it.

The addition of the Reverse Realm was basically a "New Game Plus" for the mod, since you could go to it and face the overleveled challenges, to try getting the gears it had to offer to you. It was created with part of the idea of the reverse castle from Castlevania Symphony of the Night, but I guess I should have made so you could only enter it through the Underworld, since It has a roof.