Adult Prismatic Dragon

Bardic Inspiration (7/day). As a bonus action, the dragon chooses another creature that can see or hear it within 120 feet. That creature gains a bardic inspiration die if it does not already have one, a d12. Before 10 minutes pass, the creature can expend the die to add its result to an attack roll or saving throw.

Empowering Reserves (1/day). When making an ability check, the dragon can tap into a sorcerous font of magic to gain advantage on the check.

Innate Spellcasting. The dragon’s innate spellcasting ability is Constitution (spell save DC 22). The dragon can cast prismatic spray (see actions below).

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

Song of Rest. The dragon can use soothing music or oration to help revitalize its wounded allies during a short rest. If the dragon or any friendly creatures who can hear its performance regain hit points by spending Hit Dice at the end of the short rest, each of those creatures regains an extra 1d10 hit points.

Spellcasting. The dragon is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). The dragon knows the following bard and sorcerer spells:

Cantrips (at will): acid splash, mage hand, minor illusion, poison spray, prestidigitation, ray of frost, shocking grasp, vicious mockery

1st level (4 slots): burning hands, color spray, ice knife, identify, illusory script, longstrider, speak with animals, unseen servant 

2nd level (3 slots): blindness/deafness, heat metal

3rd level (3 slots): dispel magic, major image

4th level (3 slots): compulsion, dimension door

5th level (2 slots): cloudkill, cone of cold, geas

6th level (1 slot): true seeing

7th level (1 slot): delayed blast fireball, fire storm, forcecage

8th level (1 slot): power word stun

9th level (1 slot): (no spells known but can be used to cast lower level spells with a 9th-level spell slot)

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 3 (1d6) acid damage plus 3 (1d6) cold damage plus 3 (1d6) fire damage plus 3 (1d6) lightning damage plus 3 (1d6) poison damage.

Claw. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 16 (2d6 + 9) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on 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, 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 dragon's Frightful Presence for the next 24 hours.

Prismatic Breath (Recharge 5–6). The dragon innately casts prismatic spray. Its spellcasting ability for this spell is Constitution (spell save DC 22).

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

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Spell (Costs 1-3 Actions). The dragon casts a spell of 3rd-level or lower, spending one legendary action per level of the spell or one on a cantrip.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 16 (2d6 + 9) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.