Crystal Slasher

Elders Awakening (Recharges after a Short or Long Rest). If the slasher would be reduced to 0 hit points, its current hit point total instead resets to 500 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the slasher can now use its mythic actions during the next hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the slasher after its Gem Awakening activates.

Death Throes. When the slasher dies, it explodes, and each creature within 120 feet of it must make a DC 27 Dexterity saving throw, taking 70 (20d6) force damage on a failed save, or half as much damage on a successful one. The explosion destroys objects in that area that aren’t being worn or carried, and it destroys the slasher's body.

Legendary Resistance (5/Day). If the slasher fails a saving throw, it can choose to succeed instead.

Unusual Nature. The slasher doesn’t require food or drink.

Flyby. The slasher doesn't provoke opportunity attacks when she flies out of an enemy's reach.

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

Magic Weapons. The slasher’s weapon attacks are magical.

Multiattack. The slasher makes one Bite attack and two Claw attacks and one tail attack.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) piercing damage plus 27 (5d10) force damage.

Claw. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 28 (4d8 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 21) and is restrained until this grapple ends. The slasher can have only up to four creature grappled in this way at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 50 ft., one target. Hit: 37 (6d8 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Prismatic Breath (Recharge 6). The slasher exhales rainbow energy in a 500-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw. On a failed save, roll a d20 to determine the effects.

1. Red. The target takes 20d6 fire damage on a failed save, or half as much damage on a successful one.
2. Dark Red. The target . A target takes (15d6) fire damage and be covered in glass becoming paralyzed, a paralyzed creature can make a DC 16 strength saving throw shattering the glass and ending the paralysis on a success, if a paralyzed creature takes 20 or more bludgeoning damage from one attack the glass shatters ending the paralysis and the creature takes an additional 13 (2d12) slashing damage.
3. Orange. The target takes 20d6 acid damage on a failed save, or half as much damage on a successful one.
4. Amber. The target is covered in amber that hardens on impact. wile covered in amber the creatures are paralyzed. if the amber is within 5 feet of fire or the paralyzed creature takes fire damage it shatters ending the paralysis.
5. Yellow. The target takes 20d6 lightning damage on a failed save, or half as much damage on a successful one.
6. Bright Yellow. The target takes 15d6 thunder damage and is incapacitated until the end of its next turn.
7. Green. The target takes 20d6 poison damage on a failed save, or half as much damage on a successful one.
8. Dark Green. The Target takes 15d6 psychic damage, and until the end of its next turn, when the creature makes an attack roll or an ability check, it must roll a d8 and reduce the total by the number rolled.
9. Blue. The target takes 20d6 cold damage on a failed save, or half as much damage on a successful one.
10. Dark Blue. The target takes 20d6 thunder damage on a failed save, or half as much damage on a successful one.
11. Indigo. The target takes 20d6 force damage on a failed save, or half as much damage on a successful one.
12. Violet.  The target takes 15d6 force damage, and its speed becomes 0 until the start of the slasher's next turn.
13. Pink. The target takes 20d6 psychic damage on a failed save, or half as much damage on a successful one.
14. Shining White. The target takes 15d6 radiant damage. The slasher then gains 50 temporary hit points by absorbing a portion of the radiant energy.
15. White. The target takes 20d6 radiant damage on a failed save, or half as much damage on a successful one.
16. Pure Black. the target takes 15d6 necrotic damage and is weakened until the end of its next turn. A weakened creature has disadvantage on Strength-based ability checks and Strength saving throws, and the creature’s weapon attacks that rely on Strength deal half damage.
17. Black. The target takes 20d6 necrotic damage on a failed save, or half as much damage on a successful one.
18. Gray. On a failed save, the target is restrained. It must then make a Constitution saving throw at the end of each of its turns. If it successfully saves three times, the spell ends. If it fails its save three times, it permanently turns to stone and is subjected to the petrified condition. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind.
19. Gold. On a failed save, the target is blinded. It must then make a Wisdom saving throw at the start of your next turn. A successful save ends the blindness. If it fails that save, the creature is transported to another plane of existence of the GM's choosing and is no longer blinded. (Typically, a creature that is on a plane that isn't its home plane is banished home, while other creatures are usually cast into the Astral or Ethereal planes.) 
20. Special. The target is struck by two rays. Roll twice more, rerolling any 20.

Change Shape. The slasher magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the slasher is reduced to 0 hit points or uses a bonus action to end it.

Psychic Step. The slasher magically teleports to an unoccupied space it can see within 400 feet of it.

The slasher can take 6 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 slasher regains spent legendary actions at the start of its turn.

Claw. The slasher makes one Claw attack.

Psionics (Costs 2 Actions). The slasher uses Psychic Step.

Wing Attack (Costs 2 Actions). The slasher beats its wings. Each creature within 30 feet of it must succeed on a DC 27 Dexterity saving throw or take 36 (4d12 + 10) bludgeoning damage and be knocked prone. The slasher can then fly up to half its flying speed.

Psychic Beam (Costs 2 Actions). The slasher emits a beam of psychic energy in a 90-foot line that is 10 feet wide. Each creature in that area must make a DC 27 Intelligence saving throw, taking 55 (10d10) psychic damage on a failed save, or half as much damage on a successful one.

If the slasher’s Elders Awakening trait has activated in the last hour, it can use the options below as legendary actions.

Bite. The slasher makes one Bite attack.

Arcane Spear. The slasher creates four spears of magical force. Each spear hits a creature of the slasher’s choice it can see within 200 feet of it, dealing 23 (3d8 + 10) force damage to its target, then disappears.

Energy Flare (Costs 4 Actions). The slasher flares with elemental energy. Each creature in a 120-foot-radius sphere centered on the slasher must succeed on a DC 27 Dexterity saving throw or take 54 (12d8) force damage.