You have a coin infused with the tides of chaos. Whenever you or an ally makes a D20 Test, you can flip a coin to add a bonus to the roll. The bonus equals +4 on heads, or +1 on tails.
Once you use the coin, the coin passes to the GM, who can use it to add a bonus to any D20 Test made by a creature under their control. Once the GM uses the coin, it passes back to you, and so on. When you finish a Short or Long Rest, you regain the coin, whether or not the GM has used it.