Evard's Black Tentacles

Casting time: 1 Action-Range: 90 feet
Components: V, S, M (a piece of tentacle)
Duration: Concentration, up to 1 minute
Squirming, ebony tentacles fill a 20-foot square on ground that you can see within range.
Must succeed on a Dex saving throw or take 3d6 bludgeoning damage and be restrained.
If it starts the turn in the area and is already restrained by the spell takes 3d6 bludgeoning dmg.