broken kneecaps

You curse a creature’s legs with a sudden surge of crushing force that shatters the strength of their knees. Choose a creature you can see within range. The target must make a Constitution saving throw. On a failed save, the creature takes 5d8 necrotic damage, and its kneecaps buckle under the magical pressure.

While affected, the creature’s speed becomes 0, and it cannot stand up if it is knocked prone. The creature may still crawl, but its crawling speed is 5 feet. At the end of each of its turns, the creature can repeat the saving throw, ending the effect on a success.

On a successful save, the creature takes half damage and its speed is halved until the start of your next turn.