Vinewarden's Blade

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects, and thorn vines grow on it. Until the start of your next turn, the first attack the creature makes against a target other than you causes the thorn vines to constrict and deal 1d6 piercing damage.

At Higher Levels. At 5th-level the melee attack deals an extra 1d6 piercing damage on a hit, and the damage the target takes for attacking another target increases to 2d6. Both damage rolls increase by 1d6 at 11th level (2d6 and 3d6) and again at 17th level (3d6 and 4d6).