mushroomite crown
Plantera
In the underworld there is a triangular temple that can only be entered after defeating Moon Lord . Once entered you see a statue of a rainbow on the right and on the other side you see a statue of a skull. If you walk over to the rainbow statue the statue of the skull will sink into the ground and the rainbow statue will become a real rainbow. If you go back to the surface you'll notice the hallow and corruption/crimson have disappeared and monsters spawn a lot less. If you walk over to the skull statue the rainbow statue will sink into the ground and the skull stature will catch on fire and will burn forever. If you go back to the surface you will notice the hallow and corruption/crimson have taken over all the biomes and monsters spawn twice as much. And if you want you can avoid the temple and the world stays the same.