Biome Controllers are responsible for the layout of biomes in ages created with Mystcraft. Every age description requires exactly one Biome Controller. Naturally, they take biome pages as modifiers; most have a minimum number of biomes required (and will add random ones as needed), but can accept however many biomes have been supplied. Biomes are laid out regardless of whether there is actual terrain, even a Void age will have biomes. Most of the options produce "blob" biomes similar to those found in the Overworld, but at various scales.
The options are as follows:
- Native Biome Controller: This takes no modifiers. it produces a layout similar to the Overworld, but possibly including biomes which are not actually generated in the Overworld (such as biomes from Twilight Forest or other dimension mods).
- Single Biome Controller: This takes a single biome and no more. The entire Age will consist of this biome.
- Sized Biome Controllers: There are five symbols which do essentially the same thing at different scales: Huge, Large, Medium, Small, Tiny. Each of these requires at least three biomes, but can use more. They produce biomes shaped similarly to the Overworld's, but limited to the modifiers chosen by the player or the grammar. For Small and especially Tiny biomes, the transitions between different biomes can dominate the landscape. This can make for some very strange terrain, and may prevent the biomes from reaching their full height or depth.
- Grid-Form Biome Controller: Breaking away from the blobs, this symbol accepts two or more biomes, and creates a checkerboard pattern of chunks—that is, the biome changes at each chunk boundary. As with Tiny Biomes, the transitions between dissimilar biomes can dominate the landscape.
- Tiled Biome Controller: This was the first attempt at Grid-Form, retained for compatibility and variety. It behaves similarly to Grid-Form (and likewise takes two or more biomes), but the landscape generation and decoration are buggy: Essentially, the visible surface will show a checkerboard pattern four times the size of the actual biome grid (each square being 4x4 chunks), so that most chunks get a landscape that does not match their actual biome.
- Specifically, the landscape / ground level / etc are scaled 4 times the rate of the biomes, while the actual surface decorations (trees, grass/mycelium, etc.) fit the biome of the specific chunk.