Biome Staff is a simple mod on Curseforge that adds exactly one tool: the Biome Staff. This Biome Staff can sample a biome, and then transform any area to that biome. This is a great way to create mob free zones on your server by simply painting an area to Mushroom Island biome, since mobs cannot spawn in that biome. Or maybe you want to make a Zombie Pigman farm in the overworld? You can do that too!
Biome Staff is available for Minecraft 1.12.2. Link: https://www.curseforge.com/minecraft/mc-mods/biome-staff