Source: Lost Laboratory of Kwalish

Weapon (any sword), very rare (requires attunement)

When you attack a creature with this magic weapon and roll a 20 on the attack roll, the creature must make a DC 15 Constitution saving throw in addition to suffering the attack’s normal effects. On a failed save, the creature is restrained and must make another Constitution saving throw at the end of each of its turns. If it successfully saves against this effect three times, the effect ends. If it fails its saves three times, it is turned to stone and subjected to the petrified condition for 1 hour.

A creature is immune to this effect if it is immune to damage of the weapon’s type, does not have a body made of flesh, or has legendary actions.

Curse. This weapon is cursed, and becoming attuned to it extends the curse to you. Until the curse is broken with a Remove Curse spell or similar magic, you are unwilling to part with the weapon. Whenever you attack a creature with this weapon and roll a 1 on the attack roll, you must succeed on a DC 15 Constitution saving throw or be restrained and forced to make additional saves against being petrified, as above.