Dark Magic Stone (5e Equipment)
From D&D Wiki
Wondrous item, legendary (requires attunement)
A black magic stone that contains dark energy, which will corrupt anyone who holds it. They are very rarely found in the stomach of monstrosities that have eaten sentient humanoids.
Curse. This item is cursed, a fact that is revealed only when an identify spell is cast on the item or you
attune to it. Attuning to it curses you until you are targeted by the remove curse spell or similar magic. Removing the stone from your body fails to end the curse on you. While cursed, at dawn, your Intelligence score decreases by 1, to a minimum of 5. At dawn, if your Intelligence score is 5 or less, you will go on a berserk rampage, attacking the nearest creature you can see, until the curse is dispelled or you are killed. Intelligence lost through the curse can be restored through a greater restoration spell.