Ancient Phoenix

Fiery Death and Rebirth. When the phoenix dies, it explodes. Each creature within 60-feet of it must make a DC 22 Dexterity saving throw, taking 26 (4d12) fire damage on a failed save, or half as much damage on a successful one. The fire ignites flammable objects in the area that aren't worn or carried. The explosion destroys the phoenix's body and leaves behind an egg-shaped cinder that weighs 5 pounds. The cinder is blazing hot, dealing 21 (6d6) fire damage to any creature that touches it, though no more than once per round. The cinder is immune to all damage, and after 1d6 days, it hatches a new phoenix.

Fire Form. The phoenix can move through a space as narrow as 1 inch wide without squeezing. Any creature that touches the phoenix or hits it with a melee attack while within 5 feet of it takes 14 (2d12) fire damage. In addition, the phoenix can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 14 (2d12) fire damage. With a touch, the phoenix can also ignite flammable objects that aren't worn or carried (no action required).

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

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

Innate Spellcasting. The pheonix's innate spellcasting ability is Wisdom (spell save DC 22, +12 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: wall of fire, fireball

3/day each: flame strike, conjure minor elementals

1/day each: conjure elemental (fire elemental only), fire storm, wish, meteor swarm, plane shift

Illumination. The phoenix sheds bright light in a 90-foot radius and dim light for an additional 60 feet.

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

Siege Monster. The phoenix deals double damage to objects and structures.

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

Beak. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 15 (3d6 + 10) fire damage. If the target is a creature or a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 14 (2d12) fire damage at the start of each of its turns.

Fiery Talons. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 23 (3d8 + 10) fire damage.