Source: Antihero's Handbook, pg(s). 30
A Desna's coin is made of gold and has two blank faces. The coin's owner can cause the holy symbol of Desna—a butterfly with suns, moons, and stars on its wings—to appear on either face at will. This proves especially useful when performing coin flips, as the owner of a Desna's coin can essentially guarantee that the result will go in their favour.
As one would expect of an item associated with the goddess of luck, a Desna's coin also confers on its owner an uncanny ability to fortuitously avoid sudden threats by enhancing the bearer's reflexes. A Desna's coin costs around 8,000 gold pieces.