Ancient Thunderbird

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

Empowered Attacks. The Thunderbird's weapon attacks are treated as magical for the purpose of overcoming resistance and immunity to damage from nonmagical attacks.

Flyby. The Thunderbird doesn’t provoke opportunity attacks when it flies out of an enemy’s reach.

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

Storm Aura. The air around the Thunderbird is constantly churning with fierce winds, cracks of thunder and occasional bolts of lightning. The area within 30 feet of the Thunderbird is considered difficult terrain, and any unprotected, non-magical flame within is extinguished. Any non-magical ranged attack targeting a creature in this area is made at disadvantage.

Innate Spellcasting. The Thunderbird's innate spellcasting ability is Wisdom (spell save DC 20, +12 to hit with spell attacks). The Thunderbird can innately cast the following spells, requiring no material components:

At will: Fog Cloud

3/day each: Lightning Bolt, Shatter(4th level)

2/day each: Chain Lightning, Control Weather

1/day each: Earthquake

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

Beak. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 27 (4d8 + 9) piercing damage.

Talons. Melee Weapon Attack: +16 to hit, reach 5 ft., one target. Hit: 23 (4d6 + 9) slashing damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the Thunderbird can't use its talons on another target.

Lightning Storm (Recharge 6). All other creatures within 120 feet of the Thunderbird must each make a DC 20 Dexterity saving throw, taking 27 (6d8) lightning damage on a failed save, or half as much damage on a successful one. If a target’s saving throw fails by 5 or more, the creature is also stunned until the end of its next turn.

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

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

Lightning Strike. The Thunderbird can cause a bolt of lightning to strike a point on the ground anywhere under its storm. Each creature within 5 feet of that point must make a DC 20 Dexterity saving throw, taking 16 (3d10) lightning damage on a failed save, or half as much damage on a successful one.

Wing Attack (Costs 2 Actions). The Thunderbird beats its wings. Each creature within 10 feet of the Thunderbird must succeed on a DC 20 Dexterity saving throw or take 16 (2d6 + 9) bludgeoning damage and be knocked prone. The Thunderbird can then fly up to half its flying speed.