Tentacle of the Depths

As a bonus action, you can summon a tentacle to restrain your foes. They must succeed on a Dexterity saving throw, DC equal 8+ twice your proficiency, or be restrained by the tentacle for 1 minute. A restrained creature can repeat the saving throw at the end of each of their turns, ending the effect on a success. You must concentrate on this ability as you could concentrate on spell. If a creature succeeds on the save you can force them to reroll the saving throw potentially causing it to fail. To do so you sacrifice a number of hit points equal to the CR or level of the creature you are fighting.

Once you've used this ability you cannot use it again until you complete a short or long rest.