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Marauder's Arch is a location in Sea of Thieves. Received a quest from a bottle on one of the islands (forgot which). It was named Maurders Arch Champion. I defeated the champion solo after several waves. He dropped a glowing skull I picked it up. Not sure what it does or did. Unfortunetly I had to log off to put my son to bed.
The skulls have no utility, they can only be sold for gold.