Algonquian Thunderbird

Keen Sight. The Thunderbird has advantage on Wisdom ( Perception) checks that rely on sight.

Elemental Demise. If the Thunderbird dies, its body disintegrates into a cool breeze,  dissipating the thunder clouds to reveal a clear sky.

Multiattack. The Thunderbird makes two attacks: one with its beak and one with its talons.

Beak. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 27 (4d8 + 9) piercing damage.

Talons. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 23 (4d6 + 9) slashing damage, and the target is grappled (escape DC 19). Until this grapple ends, the target is restrained, and the Thunderbird can't use its talons on another target.

Once a target is grappled, on the next turn the Thunderbirds likes to drop its prey at great heights to cause extra damage. (3d20 +10)ft to decide the height. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell,  The creature lands prone, unless it avoids taking damage from the fall.

 The Thunderbird can take 4 legendary actions, choosing from the options below as they become  available as his storm intensifies. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Thunderbird regains spent legendary actions at the start of its turn.

Round 1. A churning storm cloud forms, centered on the Thunderbird and spreading to a radius of 360 feet. Lightning flashes in the area, thunder booms, and strong winds roar. Each creature under the cloud (no more than 5,000 feet beneath the cloud) when it appears must make a Constitution saving throw. On a failed save, a creature takes 2d6 thunder damage and becomes deafened for 5 minutes.

Each round the storm intensifies, and produces additional legendary effects. Only when the storm has gone through all four rounds can  the Thunderbird randomly pick which legendary action to be used. 

Round 2. The Thunderbird calls six  bolts of lightning from the cloud to strike six creatures or objects of its choice beneath the cloud. A given creature or object can't be struck by more than one bolt. A struck creature must make a Dexterity saving throw. The creature takes 10d6 lightning damage on a failed save, or half as much damage on a successful one.

Round 3. Hailstones rain down from the cloud. Each creature under the cloud takes 2d6 bludgeoning damage.

Round 4. Gusts and freezing rain assail the area under the cloud. The area becomes difficult terrain and is heavily obscured. Each creature there takes 1d6 cold damage. Ranged weapon attacks in the area are impossible. The wind and rain count as a severe distraction for the purposes of maintaining concentration on spells. Finally, gusts of strong wind (ranging from 20 to 50 miles per hour) automatically disperse fog, mists, and similar phenomena in the area, whether mundane or magical.