Ancient Abyssal Dragon

Abyssal Devotion: As an action on its turn, the dragon can channel Abyssal energy and cast one of the following spells (save DC 21);

At WIll (at 4th level): Magic Missile, Thunderwave

(3/day) (6th level): Circle of Death, Harm

(1/day) (8th level): Hellfire Orb

Blindsight: The dragon can naturally perceive objects and creatures within a 100 foot area. This includes seeing through invisibility and darkness spells, but it cannot see through solid material.

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

Multiattack: The dragon uses its Frightful Presence. It then uses its breath attack or makes four attacks; two with its claws, one with its tail and one with its bite.

Frightful Presence: Any creature that can see the Abyssal Dragon must make a DC 20 Wisdom saving throw or become Frightened for one minute. A creature frightened by this manner must use its action to move as far away as possible, and any attacks while in view of the dragon have disadvantage. A creature that successfully resists the effect is immune to the Frightful Presence for 24 hours.

Claw: Melee attack, +14 to hit, reach 10 ft. one target. Hit: 24 (3d10 + 8) slashing damage. A creature must then succeed a DC 20 Constitution saving throw or take an additional 18 (4d8) necrotic damage.

Tail: Melee attack, +12 to hit, reach 10 ft. one target. Hit: 17 (2d10 + 6) bludgeoning damage.

Bite: Melee attack, +14 to hit, reach 5 ft. one target. Hit: 24 (3d10 + 8) piercing damage and 22 (4d10) necrotic damage. A creature hit with this attack must make a DC 21 Athletics/Acrobatics check or become Grappled by the dragon. As an attack, the dragon can choose to Swallow the creature if it is Large or smaller.

Swallow: The dragon swallows a creature it had grappled with it's bite attack. While swallowed, the creature is considered Restrained, and takes 22 (5d8) acid damage at the beginning of each of their turns. If the dragon suffers more than 40 points of damage from within, or 100 points of damage from outside in a round of combat, it must make a DC 24 Constitution saving throw or regurgitate any Swallowed creatures.

Shadow Breath (Recharge 5-6): A plume of black smoke envelopes a cone 40 feet in front of the dragon. Each creature inside must make a DC 22 Constitution saving throw or take 65 (10d12) necrotic damage on a failed save, half as much on a success. If a creature is reduced to 0 hit points this way, that creature dies immediately.

The Abyssal Dragon has 4 legendary actions it can use per round.

Attack (1 action): The dragon makes a claw attack

Cast (2 actions): The dragon uses one of its spells.

Swallow (2 actions): The dragon swallows a creature it had grappled with its bite attack.

Wing Beat (3 actions): The dragon beats its wings, creating a massive whirlwind. Each creature in a 20 ft. range from the dragon must make a DC 21 Strength saving throw or be thrown 30 ft. in a random direction, taking 21 (6d6) bludgeoning damage on a failed save.