Cosmic Horror Elder

Eldritch Nature. The horror has advantage on saving throws against spells and magical effects. Any critical hits made by spell attacks against the horror become normal hits. In addition, no spell or magic short of a cult gathering or a wish spell can summon the horror.

Legendary Resistance (3/Day). If the horror fails a saving throw, it can choose to succeed instead. 

Unfathomable. Each creature within 80 feet of the horror has disadvantage on Wisdom ability and skill checks. In addition, a creature that attempts to create a telepathic link with the horror or read its mind must succeed on a DC 24 Wisdom saving throw or take 21 (6d6) psychic damage.

Multiattack. The horror makes one Bite attack and two Tentacle attacks.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 22 (4d6 + 8) piercing damage.

Tentacle. Melee Weapon Attack: +15 to hit, reach 60 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage. If the target is a creature, it is grappled (escape DC 18) and is restrained until this grapple ends. The horror has 2d8 + 4 tentacles, each of which can grapple one target.

Psychic Whispers (Recharge 5–6). The horror emits incomprehensible babble. Each creature within 120 feet of the horror must make a DC 24 Wisdom saving throw. On a failed save, the creature takes 42 (12d6) psychic damage and is deafened until the end of the horror's next turn. On a successful save, the creature takes half as much damage.

Psychic Gaze (4/Day). The horror incomprehensibly looks towards any number of creatures it can see within 120 feet of it. The targets must succeed on a DC 24 Wisdom saving throw or become incapacitated for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

The horror can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The horror regains spent legendary actions at the start of its turn.

Teleport. The horror teleports, along with any creatures it is grappling, to an unoccupied space it can see within 240 feet of itself.

Crush (Costs 2 Actions). The horror crushes one creature it is grappling. The creature takes 15 (2d6 + 8) force damage.

Terror Shriek (Costs 2 Actions). The horror shrieks covering a 60-foot-sphere radius. Each creature in this area must succeed on a DC 24 Wisdom saving throw or fall unconscious. A lesser restoration spell or similar magic awakens the creature.