In my Tests it seems to be a global +stat that increases the overall Chance of a further Wave being a cultist Wave. My max was so far +42% but it hadn't much of effect... possibly I was just unlucky or there is a hidden cap for max +cultist Wave...
Note that cultist waves seem not to occur during Bonus rounds just during normal game... possibly via Necronomicon they could be summoned but i wasnt able to Test yet.
Andy posted on Discord that the base chance is 16% and the max spawn rate is 50%.
WRONG: He also said the items don't need to be equipped to trigger the increased spawn rate, which surprised me.
Edit: added "Wrong" before my false statement. I misunderstood what Andy said.
Just to clarify, the item needs to be equipped for spawn chance to have an effect, it just doesn't matter which tower.
The Discord link is in the Mazebert app at the top of the multiplayer screen.
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