Polluted Earth is a block which spreads, replacing other blocks. Also it spawns specified entities.
Entities which can be spawned is configurable.
Blocks which can be replaced is configurable.
Rate of spread and rate of mob spawning are configurable.
The block does its job on random tick.
Initially, the array of spawnable entities and replaceable blocks are empty - define them by adding full registry names in mod settings.
Polluted Earth is located in creative tab "Decorations".