All or Nothing

The All or Nothing item appears to be a prismatic platinum coin with a bleeding eyepatch for heads and a scale for tails. 

This coin can be used as a reaction to replace a rolled d20 . The player then calls heads or tails; if it is correct, it becomes a critical success and if it fails, it becomes a critical fail. If the result is failure, then its an automatic critical fail, no matter proficiency bonus The coin cannot be used in all scenarios, which can be decided by the Dm. It costs 10 Gp to use and can only be used twice per long rest. It is regarded as legendary among gamblers, rogues and vagrants due to the item being the coin being used by the Gambler of fate, Solana Fistspark, the celebrated mayor, who became a revered pirate because she one day chose to live her life on the flip of a coin. 

"It shows how easily one can win and just how easily it is to lose." -Solana Fistspark