Behogrillacer beast

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

Magic Resistance. The behogrillacer beast has advantage on saving throws against spells and other magical effects.

Avoidance. If the behogrillacer beast is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Innate Spellcasting. The Behogrillacer beasts spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). The Behogrillacer beast can innately cast the following spells, requiring no Material components:
At will: darkness, detect magic, dispel magic 
1/day each: confusion, Maddening Darkness, power word stun 

Disrupting Gaze. The behogrillacer beast's central eye creates an area of disruption in a 150-foot cone. When a creature is hit with damage it would normally be immune to, it is instead resistant to the damage. In addition, if a creature is hit by damage that is normally resistant to, it instead takes normal damage. This affects class features, including Magic Resistance, and spells.

Displacement. The behogrillacer 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 behogrillacer beast is incapacitated or has a speed of 0.

Multiattack. The behogrillacer beast uses its Frightful Presence and makes two punches, two tentacle attacks, and one random eye ray.

Frightful Presence. Each creature of the behogrillacer beast's choice within 120 feet of it and aware of it must succeed a DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the behogrillacer beast is within line of sight, 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 behogrillacer beast's Frightful Presence for the next 24 hours.

Fist. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage.

Tentacle. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 13 (1d6 + 10) bludgeoning damage plus 3 (1d6) piercing damage.

Eye Rays. The behogrillacer beast shoots one of the following magical eye rays at random, choosing one target it can see within 120 feet of it:

1. Charm Ray. The targeted creature must succeed on a DC 21 Wisdom saving throw or be charmed by the behogrillacer beast for 1 hour, or until the behogrillacer beast harms the creature.
2. Paralyzing Ray. The targeted creature must succeed on a DC 21 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
3. Fear Ray. The targeted creature must succeed on a DC 21 Wisdom saving throw or be frightened for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
4. Slowing Ray. The targeted creature must succeed on a DC 21 Dexterity saving throw. On a failed save, the target's speed is halved for 1 minute. In addition, the creature can't take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
5. Enervation Ray. The targeted creature must make a DC 21 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one.
6. Telekinetic Ray. If the target is a creature, it must succeed on a DC 21 Strength saving throw or the behogrillacer beast moves it up to 30 feet in any direction. It is restrained by the ray's telekinetic grip until the start of the beholder's next turn or until the beholder is incapacitated.
7. Sleep Ray. The targeted creature must succeed on a DC 21 Wisdom saving throw or fall asleep and remain unconscious for 1 minute. This ray has no effect on constructs and undead.
8. Petrification Ray. The targeted creature must make a DC 21 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.
9. Disintegration Ray. If the target is a creature, it must succeed on a DC 21 Dexterity saving throw or take 45 (10d8) force damage. If this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust. If the target is a Large or smaller nonmagical object or creation of magical force, it is disintegrated without a saving throw. If the target is a Huge or larger object or creation of magical force, this ray disintegrates a 10-foot cube of it.
10. Death Ray. The targeted creature must succeed a DC 21 Dexterity saving throw or take 55 (10d10) necrotic damage. The target dies if the ray reduces it to 0 hit points.

Join The Dark Side The Behogrillacer Beast uses its Charm ray on a target within 10 feet. If the target fails, the creature grapples the target and places it on it's back as a mounted ally and spends its entire action each turn for 1 minute clinging to the Behogrillacer Beast and cheering it on, oblivious to any attack attacks it makes against its previous allies. The target can repeat the save at the end of each of its turns, ending the effect on a success.

Behold The Beast (Recharge 5-6). The behogrillacer beast uses its Paralyzing Eye on a creature within melee range. If the target fails, the behogrillacer beast immediately makes four fist attacks and two tentacle attacks on the target. If target succeeds on the save, all attacks are made at disadvantage.

The behogrillacer 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 behogrillacer beast regains spent legendary actions at the start of its turn.

Eye Ray. The behogrillacer beast uses one random eye ray.

Attack. The behogrillacer beast makes one punch or tentacle attack.

Move. The behogrillacer beast moves up to half its speed without provoking opportunity attacks.

Perpetual Darkness The creature begins emitting magical darkness that extends 15ft from the behogrillacer beast and moves with the creature. It cannot be dispelled by any magic 7th level or lower.

Doom Charge (Costs 3 Actions) The behogrillacer beast runs its speed in a single direction and is able to make an attack with all appendages for any target in the path, any creature directly in front of it must make a DC 20 Strength save or be knocked prone and take 22 (4d10) bludgeoning damage.