Source: Ghosts of Saltmarsh

Wondrous item, uncommon (requires attunement)

This flat, gray-and-black river stone is inscribed with an unknown arcane symbol and feels cool to the touch. While carrying the stone. you can gain advantage on one ability check of your choice. The stone can't be used this way again until the next dawn.

Curse. This item is cursed. Attuning to it curses you until you are targeted by a remove curse spell or similar magic. As long as you remain cursed, you cannot discard the stone, which immediately teleports back into your pocket or pack. After you use the stone's magic, your next two ability checks are made with disadvantage.