Hello! Im here cause i need some help. I am a beginner mod creator and im working on a mod that adds planets and stuff. So i was wondering if anyone knows how to replace for example air with my custom block carbon dioxide or any other gas (block) while the dimension (which is the planet) is generating so players need some oxygen gear to breathe. Thanks for any answers!
Edit: Forgot to mention the mod is for version 1.12.2.