Being of Flame.  When the Phoenix makes an attack it adds an extra 2d6 fire damage to the damage. 

Inferno. Any hostile within ten feet of the Phoenix takes 1d10 fire damage. This damage also occurs when a creature attacks the Phoenix.

Explosive Death. When the Phoenix is dropped to 0 hit points its explodes in a 20ft radius. Any creature within that radius must make a dexterity saving throw, on a fail the creature takes 10d6 fire damage or half as much on a success. 

Rebirth. Once the Phoenix dies it is reborn from the ashes it left on the material plane after falling. The process for the Phoenix to be rebirthed could take years to decades.

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

Beak. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 19 (2d8 + 3) [piercing] damage + (2d6) [fire] damage.

Talons. Melee Weapon Attack: +8 to hit, range 5 ft., 1 target. Hit: 15 (2d6 + 3) [slashing] damage + (2d6) [fire] damage and the creature is grappled (escape DC 19). Until the grapple ends the creature is restrained, and the Phoenix can't use its talons on another target.

Fire Breath (Recharge 5-6). The Phoenix exhales fire in a 60-foot line that is 10 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 60 (10d6) fire damage on a failed save, or half as much on a successful one.