Thornstrike

You imbue your weapon with the essence of nature, causing it to sprout sharp thorns. Make a melee spell attack against a creature within range. On a hit, the target takes the weapon's normal damage and are surrounded by sharp thorns.

Additionally, until the start of your next turn, if the target willingly moves 5 feet or more, they take an additional 1d8 piercing damage as the thorns dig deeper into their wounds.

At Higher Levels. At 5th level, the melee attack deals an extra 1d8 piercing damage to the target on a hit, and the damage the target takes for moving increases to 2d8. Both damage rolls increase by 1d8 at 11th level (2d8 and 3d8) and again at 17th level (3d8 and 4d8).