Altair

Amphibious. The dragon can breathe air and water.

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

Living Shadow. While in dim light or darkness, the dragon has resistance to damage that isn’t force, psychic, or radiant.

Shadow Stealth. While in dim light or darkness, the dragon can take the Hide action as a bonus action.

Innate Spellcasting. Altair is a 9th-level spellcaster. Their spellcasting ability is Intelligence (spell save DC 19, +11 to hit with spell attacks). They have the following Transmutation spells available:

Cantrips (at will): Control Flame, Gust, Shape Water, Mold Earth

1st level (4 slots): Catapult, Create/Destroy Water, Expeditious Retreat, Magnify Gravity

2nd level (3 slots): Earthbind, Heat Metal, Spike Growth

3rd level (3 slots): Blink, Erupting Earth, Slow

4th level (3 slots): Control Water, Fabricate, Giant Insect

5th level (2 slot): Animate Objects, Control Winds, Passwall

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 15 ft., one target. Hit: 21 (2d10 + 8) piercing damage.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 17 (4d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 8) 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 24 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.

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Shadow Breath. The dragon exhales shadow in a 90- foot line that is 5 feet wide. Each creature in that line must make a DC 20 Dexterity saving throw, taking 66 (12d10) necrotic damage on a failed save, or half as much damage on a successful one. A humanoid reduced to 0 hit points by this damage dies, and an undead 'Shadow'  rises from its corpse and acts immediately after Umbraxakar in the initiative count. The shadow is under Umbraxakar’s control.

Repulsion Breath. The dragon exhales repulsion energy in a 30-foot cone. Each creature in that area must succeed on a DC 20 Strength saving throw. On a failed save, the creature is pushed 60 feet away from the dragon.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Counter Elements (Recharge 5-6). As a reaction, Altair can cast 'Counterspell' without using a spell slot. This can only be done to a spell comprised of one of the four elements, Air, Earth, Fire, Water. If succeeded, the spell changes to a different element and fails. For example, a Wall of Fire may turn into water and splash and drain away.

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.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.