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

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. Any unprotected, non-magical flame within 30 feet of it is extinguished. The area within 30 feet of it is considered magical hard terrain. Any non-spell ranged attack that hits the Thunderbird has its damage reduced by 11 (2d10). If that reduces the damage to 0, the arrow is blown away and is lost.

Lightning and Thunder Regeneration. When the Thunderbird is affected by an effect that deals thunder or lightning damage, it heals that amount of hit points instead.

Death Shockwave. When the Thunderbird dies, it detonates in a shockwave of feathers. Every creature in a 30 foot radius around it must make a DC 20 Constitution Saving Throw, taking 27 (6d8) thunder damage and 22 (4d10) force damage on a failed save or half as much damage on a successful one.

Innate Spellcasting. The Thunderbird's innate spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). The Thunderbird can innately cast the following spells, requiring no material components:

At will: Fog Cloud

3/day each: Call Lightning, Shatter

2/day each: Chain Lightning

1/day each: Control Weather

Multiattack. The Thunderbird uses Frightful Presence and then makes three attacks: two with its claws and one with its bite or swallow.

Claw. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 50 (8d10 + 6) piercing damage and 14 (4d6) lightning damage. If the target is a creature, it is pulled 10 ft towards the Thunderbird.

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 27 (6d8 + 6) piercing damage and 18 (4d8) thunder damage. If the target is a creature, it is Grappled (escape DC 18). Until the grapple ends, the creature is Restrained and the Thunderbird can't attack with its bite.

Swallow. The Thunderbird makes one Bite attack against a creature it is grappling. If the attack hits, that creature takes the bite's damage, it is swallowed, and the grapple ends. While swallowed, the creature is Blinded and Restrained, it has total cover against attacks and other effects outside the Thunderbird, and it takes 21 (6d6) acid damage and 9 (2d8) thunder damage at the start of each of the Thunderbird's turns.
If the Thunderbird takes 30 damage or more on a single turn from a creature inside it, the Thunderbird must succeed on a DC 15 Constitution Saving Throw at the end of that turn or regurgitate all swallowed creatures, which fall Prone in a space within 10 feet of the Thunderbird. If the Thunderbird dies, a swallowed creature is no longer Restrained by it and can escape from the corpse using 15 feet of movement, exiting Prone.

Frightful Presence. Each creature of the Thunderbird's choice that is within 120 feet of it and aware of it must succeed on a DC 18 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 Thunderbird's Frightful Presence for the next 24 hours.

Lightning Breath (Recharge 5-6). The Thunderbird exhales a line of lightning that is 60 feet long and 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 56 (16d6) lightning damage on a failed save, or half as much damage on a successful one. If the creature fails by 10 or more, it is Stunned until the end of its next turn.

Thunder Scream (Recharge 5-6). The Thunderbird emits a burst of thunder energy in a 20 foot radius. Each creature in that sphere must succeed on a DC 18 Constitution Saving Throw or take 44 (10d8) thunder damage and be knocked Prone. On success, take half damage and not knocked prone. If the creature fails by 10 or more, it is Stunned until the end of its next turn.

Legendary Reactions.

• If a character hits the Thunderbird twice in a row with a non-spell ranged attack, the Thunderbird grabs the projectile and throws it back. (+8 to hit, 2d10 + 2 piercing damage).
• When the Thunderbird is a reduced to half of its maximum hit points or lower for the first time, it makes one Wing Attack.

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.

Move (Only if Flying). The Thunderbird moves up to its flying speed without provoking attacks of opportunity.

Wing Attack (Costs 2 Actions). The Thunderbird beats its wings. Each creature within 30 feet of the Thunderbird must succeed on a DC 18 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked Prone. The Thunderbird can then fly up to half its flying speed.

Swallow (Costs 2 Actions). The Thunderbird uses its Swallow attack on a creature it is grappling.

Lightning Breath (Costs 3 Actions). The Thunderbird recharges its Lightning Breath and uses it.

Thunder Scream (Costs 3 Actions). The Thunderbird recharges its Thunder Scream and uses it.