Feel like your forgetting this ability.that's, like, your opinion. My players would find that anti climatic for a boss fight at the end of an adventure. Also, I can't see anyway it even comes close to killing a PC unless it focuses fire on one of them.
Mind Blast (Recharge 5–6). Creatures of the refraction’s choice within 60 feet of it must succeed on a DC 19 Intelligence saving throw or take 33 (5d10 + 6) psychic damage and have the stunned condition for 1 minute. A stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.