Colossal Darkside

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

Cast from Darkness. When the Colossal Darkside drops to 0 hit points, it's body explodes and releases a swarm of Shadows. Each creature within 20 ft. of the explosion must make a DC 13 Dexterity saving throw, taking 3d12 Necrotic damage on a failed save, or half as much on a successful one. Then, 4d4 Shadows emerge from the remains of the Colossal Darkside within 20 ft. of where the Colossal Darkside died.

Innate Spellcasting. The Colossal Darkside's innate spellcasting ability is Charisma, (spell save DC 25, +15 to hit with spell attacks). The Colossal Darkside can innately cast the following spells, requiring no material components:
At will: Darkness, Ray of Enfeeblement
3/day: Confusion, Dispel Magic, Shadow of Moil
2/day: Disintegrate (9th level)

Multiattack. Darkside can use its Frightful Presence. It then makes three attacks with its claws. It can replace a claw attack with it's stomp, if it is available.

Claw Attack. Melee Weapon Attack: +17 to hit, reach 15 ft., 1 target. Hit: 25 (4d8 + 8) slashing damage plus 7 (1d12) necrotic damage. 

Stomp (Recharge 5-6). Melee Weapon Attack: +17 to hit, reach 10 ft., 1 target. Hit: 38 (5d10 + 8) bludgeoning damage plus 7 (1d12) necrotic damage. The target must succeed on a DC 22 Strength saving throw. On a failed save, the target is restrained.

All creatures within 20 ft. of the attack must succeed on a DC 19 Dexterity saving throw or fall prone.

Frightful Presence. Each creature of the Colossal Darkside's choice that is within 120 ft. of the Colossal Darkside and aware of it must succeed on a DC 22 Charisma saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Colossal Darkside's Frightful Presence for the next 24 hours.

Conjure Shadows. The Colossal Darkside conjures 2d4 Shadows, which appear within any unoccupied space within 20ft of itself.

The Colossal Darkside 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 Colossal Darkside regains spent legendary actions at the start of its turn.

Claw Attack. Colossal Darkside makes a claw attack.
Conjure Shadows (Costs 2 Actions). Colossal Darksideuses its Conjure Shadows ability.
Spellcasting (Costs 2 Actions). Colossal Darkside casts one of its at-will spells.
Dark Meteors (Costs 3 Actions). Colossal Darkside calls forth a hail of meteors made of dark energy. Choose 6 creatures within 60 ft. Each creature must succeed on a DC 22 dexterity saving throw, taking 16 (4d8) force damage on a failed save or half as much on a successful one.