The world should have a certain stage that is on, and when you kill a certain boss, the stage increases. Different mobs spawn only on certain stages. When the Juggernaut is killed, the stage should go from 1 (default) to 2, and a creepy message shows up in chat (this is going to be funny). Also, the Devil Sheep spawns only in stage 2. Maybe a new command to change stage? Also, GUI to show stage, maybe in inventory (if possible)?

Devil sheep will spawn in stage 1, but other, more powerful mobs will spawn in stage 2

Also, if someone sees the "help wanted" label, I need help making the GUI once I implement the system

Need help adding this in the first place, it seems to reset every time the game is relaunched

Not going to add the command