Wondrous item, rare
This ancient platinum coin has runic letters spelling out "Fate" on one side, and XIII representing the number 13 in Roman numerals under a rounded image of a full bag on the other. It has a nice weight for flipping, and it rings out a clear tone whenever flipped, easily audible within 60 feet.
As a reaction, you may flip the coin, requiring at least one free hand or a mage hand cast by someone who wields wild magic. The result of any d20 roll within 30 feet of the coin can be reversed, by flipping the die over and reading the bottom face. For normal d20s, this should result in a new value of 21 minus the original roll. So a roll of 1 would become a 20 and vice versa.