Stone of Ill Luck

From Tales From the Yawning Portal, page 229.

Description
Wondrous item, uncommon (requires attunement).

This polished agate appears to be a Stone of Good Luck to anyone who tries to identify it, and it confers that item's property while on your person.

Curse. This item is cursed. While it is on your person, you take a -2 penalty to ability checks and saving throws. Until the curse is discovered, the DM secretly applies this penalty, assuming you are adding the item's bonus. You are unwilling to part with the stone until the curse is broken with Remove Curse or similar magic.