Grasping Tentacle

Level 10 Fathomless Warlock

At 10th Level, you gain resistance to cold, lightning, or thunder damage. At the start of each morning and roll of innitiative, you may choose which resistance you want.
  You also learn the spell Evard's Black Tentacles. It counts as a warlock spell for you, but it doesn’t count against the number of spells you know. You can also cast it once without a spell slot, and you regain the ability to do so when you finish a long rest.
    Whenever you cast this spell, your patron’s magic bolsters you, granting you a number of temporary hit points equal to your warlock level. Moreover, damage can’t break your concentration on this spell.