Belgarath

Mask trades.  Belgarath has a Bard mask and a Monk mask. While wearing one of these masks, he is granted different benefits.

Bard Mask.
Belgarath increases his Charisma by 5 (to 20)
Gains resistance to [Psychic] and [Force] damage
Gains advantage on saving throws against spells.

Monk Mask.
Belgarath increases his Dexterity by 5 (to 20)
Increases the power of his Slam attack
Increases his AC by 3 (to 20)
Gains resistance to [Bludgeoning] and [Fire] damage

Multiattack.  Belgarath makes two Slam attacks with his Great-Flute.

Slam. Melee Weapon Attack: +5 (+8 while Monk) to hit, reach 5 ft., 1 target. Hit: 12 (2d6 + 5) [bludgeoning] damage. (Hit: 15 (2d6 + 8) while Monk)

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Bard Mask.  While wearing the Bard Mask, Belgarath makes two Slam attacks with his Great-Flute, and uses one Magic ability. Or, he uses two magic abilities.

Charming Melody. Belgarath chooses one target he can see within 30 ft. That target musk succeed a DC:18 Wisdom saving throw. On a failed save, they become charmed by Belgarath. 
A creature charmed by this effect can't take actions that directly harm Belgarath, as long as he's wearing the Bard Mask. At the start of their turn, they may repeat the DC:18 saving throw, ending the effect on a success.
Only one creature can be charmed by Belgarath at any time. If a new character becomes charmed, the effect ends for any previously charmed creature.
When the effect ends for whatever reason, the affected character becomes stunned until the end of their next turn, unless they succeed a DC:18 Wisdom saving throw.

Dance for Me. Belgarath chooses a character charmed by him. He can make that character move a distance equal to their movement speed, and force them to take an attack action against a target of his choice, that does not use a spell slot greater than 2nd level.

Song of Self (1/day). Belgarath gets 5d6 temporary hit points, and if he's Stunned, Charmed, Poisoned, Blinded and or Deafened, those conditions are all removed. (This ability can be used while Stunned)

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Monk Mask.  While wearing the Monk Mask, Belgarath makes three Slam attack. Or one Slam and one other attack.

Toxic Blow Dart. Ranged Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 8 (1d6 + 5) [Piercing] damage. The hit creature must succeed a DC:15 Constitution saving throw, or gain a Venom Counter, and take 1d4 [poison damage] at the start of each of their turns for 1 minute.
A creature already poisoned by this effect is immune to a second instance of it.

Ground Slam (1/day). Belgarath Slams his Great-Flute into the ground, creating a shock wave that pushes every creature and light object within a 5 ft. radius, 20 ft back. Each creature pushed back this way must make a DC:15 Strength saving throw. On a failed save, they are knocked Prone and take 4d6 [bludgeoning] damage. On a success, they take half the damage.
Any creature who collides with an object on the path has disadvantage on the saving throw.

Enter the description for how legendary actions work for your monster here.

Masquarade (Costs 1 Actions). Belgarath switches to, or puts on one of his masks.

Melting of the Masks. When Belgarath is knocked to 0 hit points, if his body is not disintegrated, the masks melts together against his skin, and he rises once more for one last stand.

He regains 60 hit points, resets the uses of his 1/day abilities, and until defeated he gets each benefit of both the Bard and the Monk mask.
Belgarath increases his Charisma and Dexterity by 5 (both to 20)
Gains resistance to [Psychic], [Force], [Bludgeoning] and [Fire] damage
Increases the power of his Slam attack
Increases his AC by 3 (to 20)
Gains advantage on saving throws against spells.

Mask Melted Multiattack. While Mythic, Balgarath makes two Slam attacks. Then the can chose to either do another slam attack or a Bard ability. Then repeat the same process for a Monk ability.