Fantasy is a library that allows for dimensions to be created and destroyed at runtime on the server. It supports both temporary dimensions which do not get saved, as well as persistent dimensions which can be safely used across server restarts.

To add Fantasy to your Gradle project, add the Nucleoid Maven repository and Fantasy dependency. FANTASY_VERSION should be replaced with the latest version from Maven.

repositories {
  maven { url = 'https://maven.nucleoid.xyz/' }
}

dependencies {
  // ...
  modImplementation 'xyz.nucleoid:fantasy:FANTASY_VERSION'
}

All access to Fantasy's APIs goes through the Fantasy object, which can be acquired given a MinecraftServer instance.

Fantasy fantasy = Fantasy.get(server);
// ...

All dimensions created with Fantasy must be set up through a RuntimeWorldConfig. This specifies how the dimension should be created, involving a dimension type, seed, chunk generator, and so on.

For example, we could create a config like such:

RuntimeWorldConfig worldConfig = new RuntimeWorldConfig()
        .setDimensionType(DimensionType.OVERWORLD_REGISTRY_KEY)
        .setDifficulty(Difficulty.HARD)
        .setGameRule(GameRules.DO_DAYLIGHT_CYCLE, false)
        .setGenerator(server.getOverworld().getChunkManager().getChunkGenerator())
        .setSeed(1234L);

Values such as difficulty, game rules, and weather can all be configured per-world.

Once we have a runtime world config, creating a temporary dimension is simple:

RuntimeWorldHandle worldHandle = fantasy.openTemporaryWorld(worldConfig);

// set a block in our created temporary world!
ServerWorld world = worldHandle.asWorld();
world.setBlockState(BlockPos.ORIGIN, Blocks.STONE.getDefaultState());

// we don't need the world anymore, delete it!
worldHandle.delete();

Explicit deletion is not strictly required for temporary worlds: they will be automatically cleaned up when the server exits. However, it is generally a good idea to delete old worlds if they're not in use anymore.

Persistent dimensions work along very similar lines to temporary dimensions:

RuntimeWorldHandle worldHandle = fantasy.getOrOpenPersistentWorld(new Identifier("foo", "bar"), config);

// set a block in our created persistent world!
ServerWorld world = worldHandle.asWorld();
world.setBlockState(BlockPos.ORIGIN, Blocks.STONE.getDefaultState());

The main difference involves the addition of an Identifier parameter which much be specified to name your dimension uniquely.

Another very important note with persistent dimensions is that getOrOpenPersistentWorld must be called to re-initialize the dimension after a game restart! Fantasy will not restore the dimension by itself- it only makes sure that the world data sticks around. This means, if you have a custom persistent dimension, you need to keep track of it and all its needed data such that it can be reconstructed by calling getOrOpenPersistentWorld again with the same identifier.