Evard's Black Tentacles

Squirming void-black tendrils fill a 20-ft. square on the ground within range. The area is difficult terrain. A creature that enters for the first time on a turn or starts there must make a STR save, taking 4d6 bludgeoning damage and becoming restrained on a failed save, or half damage only on a success. A restrained creature can use an action to attempt a STR check against your power DC to escape.