GlobalKeyFix

This mod sets new global keys for all of the bosses and then sets the new keys to the traders to unlock their items.
This will prevent spawning of higher monsters in lower biomes once bosses are defeated.
Other mods that can do something similar: Cllc, WorldAdvancementProgression
If you are adding this to an existing world, an admin will need to check the server for existing keys and remove manually with the listkeys and removekey commands

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