Beast of a Thousand Moons

Amphibious.
The beast can breathe air and water.

Legendary Resistance (1/Day).
If the beast fails a saving throw, it can choose to succeed instead.

Manic Presence.

Each creature of the beast’s choice that is within 20 ft. and aware of it must succeed on a DC 16 Wisdom saving throw or have disadvantage on all Wisdom saving throws for 1 minute, as they are tormented by mad voices. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the beast’s Manic Presence for the next 24 hours. Any creature with a Wisdom score of 8 or lower automatically succeeds on the saving throw.

Tharizdun’s Maddening Reflection.
The beast projects a magical illusion that makes it appear to be standing near its actual location, causing attack rolls against it to have disadvantage. If it is hit by an attack, this trait is disrupted until the end of its next turn. This trait is also disrupted while the beast is incapacitated or has a speed of 0. A creature which can see the beast that has a Wisdom score of 8 or lower sees through this illusion and can attack it normally.

Pounce.
If the beast moves at least 20 feet straight toward a creature and then hits it with a claw attack on the same turn, that target must succeed on a DC 13 Strength saving throw or be knocked prone. If the target is prone, the beast can make one bite attack against it as a bonus action.

Multiattack.
The beast makes two attacks, one with its claws and one with either its bite or tentacles.

Claws.
Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage.

Bite.
Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) psychic damage, and the target's Wisdom score is reduced by 2 (1d2). The target dies if this reduces its Wisdom score to 0. Otherwise, the reduction lasts until the target finishes a long rest.

Tentacle.
Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 7 (1d8 + 4) psychic damage, and the target's Wisdom score is reduced by 2 (1d2). The target dies if this reduces its Wisdom score to 0. Otherwise, the reduction lasts until the target finishes a long rest.

Glare of A Thousand Moons (Recharge 4–6).
The beast summons the mind-rending stare of a thousand eyes that burn directly into a target's soul. The beast can force one creature it can see to make a DC 16 Wisdom saving throw, taking 12 (4d6) psychic damage on a failed save, or half as much damage on a successful one. On a failed save, the creature then rolls a d100 on the Short Term Madness Table (DMG pg 259) and adds the rolled trait to their character for 1 minute. If the saving throw fails by 5 or more the creature rolls on the Long Term Madness Table (DMG pg 260) and gains the rolled trait for 1 minute. If the saving throw fails by 10 or more the creature rolls on both the Long Term Madness Table and the Indefinite Madness Table (DMG pg 260), gaining the Long Term trait for 1 minute and the Indefinite Trait permanently (at DM discretion, there may be a way to cure it). As an action a creature can clear its mind, ending any non-permanent effects.

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

Claw.
The beast makes a claw attack.

Blink.
The beast magically teleports up to 20 feet to an unoccupied space it can see. If a creature that can see the beast has a Wisdom score of 8 or lower it sees the beast bounding through the Ethereal Plane during this movement.

Lament of the Demented (Costs 2 Actions).
The beast lets out a demented, musical growl. A creature that is deafened is immune to this effect. Any creature that can hear the growl must make a DC 13 Wisdom saving throw. On a failed save, the creature is subjected to the "Confusion" spell until the end of its next turn (no concentration required by the beast). While under the effect of that confusion, the creature is immune to Lament of the Demented.