You specify a 20-foot square area that you can see within the cast range, so that it is covered with creeping black tentacles, which makes the area difficult terrain. When any creature enters the area for the first time in a round, or starts its turn in the area, it must make a Dexterity save.
Evard’s Black Tentacles 5e
Casting Time: 1 Action
Range: 90 feet
Components: V, S, M
Duration: Concentration, up to 1 minute
Those who fail the saving throw will suffer 3d6 points of bludgeoning damage and be bound by the tentacles until the spell ends. When the creature starts its turn in the area, if it is already bound by the tentacles, it takes 3d6 points of bludgeoning damage.
A creature bound by tentacles can use an action to make a power check or a Dexterity check (choose one) against the DC of the spell save. If the check succeeds, it breaks free.