Unfathomable Horror

Innate Spellcasting. The Unfathomable Horror casts one of the spells below at its base level. Its spellcasting ability is Intelligence and it has a +9 bonus for spell attacks. The DC for saving throws against the Unfathomable horror is 17. 

At Will: Detect Thoughts, Dissonant Whispers, Shield

1/day: Confusion, Hold Monster, Phantasmal Killer, 

3/day: Bestow Curse, Command, Modify Memory

Psychic Disguise. The Unfathomable Horror warps the minds around it and impersonates a humanoid figure. A suspicious creature can succeed on a DC 23 Wisdom Insight Check to discern the Unfathomable Horror for its true nature. The Horror will detect this (whether the check is a success or not) and attempt to modify the creature's memory. 

Magic Resistance. The Unfathomable Horror has advantage on saving throws against magic, and spell attacks against it have disadvantage. 

Mental Shield. The Unfathomable Horror can add its Intelligence modifier plus its Proficiency Bonus to its AC instead of Dexterity.

Unfathomable Attacks. The Unfathomable Horror may choose to use its Intelligence modifier instead of its Strength while making attacks.

Unfathomable Aura. Creatures that start their turn within 10 feet of the Unfathomable Horror must make a DC 13 Wisdom saving throw or take 11 (1d8 + 6) psychic damage and become paralyzed until the end of their next turn as a cacophony of whispers fills their head.

Unnatural Nature. The Unfathomable Horror does not need to eat, breathe, or sleep.

Multiattack. The Unfathomable Horror makes three attacks: two with its pseudopods and one with its Mind Blast, or one with its Dissonant Whispers and one with its pseudopods. 

Pseudopod. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 11 (1d8 + 6) bludgeoning damage plus 16 (3d8) psychic damage.

Mind Blast. Ranged Spell Attack: +9 to hit, reach 30 ft. cone. Hit: 36 (6d8 + 6) psychic damage. 

Rend Sanity (Recharge 6). Ranged Spell Attack: +9 to hit, reach 60 feet, one target. Hit: 24 (4d8 + 6) psychic damage. The target must make a DC 22 Wisdom saving throw or have their sanity ripped from them as they gain one level of exhaustion and two levels of madness.

Spell Reflection. If the Unfathomable Horror makes a successful saving throw against a spell, or a spell attack misses it, the Unfathomable Horror can choose another creature (including the spellcaster) it can see within 30 feet of it. The spell targets the chosen creature instead of the Unfathomable Horror. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.