Boss Encounter 13 - lvl 5 to 10

1x Corrupted Phoenix Gargantuan monstrosity , neutral
Armor Class 18 Hit Points (10d20 + 80) Speed 20 ft., fly 120 ft.
STR 26 (+8) DEX 19 (+4) CON 25 (+7) INT 2 (-4) WIS 12 (+1) CHA 11 (+0)
Saving Throws DEX +8, CON +11, WIS +5 Damage Resistances Bludgeoning, Piercing, and Slashing from Nonmagical Attacks Damage Immunities Poison Condition Immunities Exhaustion, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained, Stunned Senses Darkvision 60 ft., Passive Perception 15 Languages -- Challenge 12 (8,400 XP) Proficiency Bonus +4
Aging Gaze. Each round, any creature who meets the Phoenix's gaze must make a save DC 13 WIS or be stricken with sudden aging 1d20 years older. A greater restoration spell can restore a creature’s age to normal. Creatures immune to aging effects (such as those who have the Timeless body ability) are immune to this effect. This is an aging effect.
Chatter Aura. The Phoenix can begin to whisper madness to those nearby. All creatures that begin its turn within 5 ft of the Phoenix must make a saving throw DC 13 WIS or become confused like the confusion for as long as they remain within 5 ft of the Phoenix. A creature confused by the Chatter is immune to Aging Gaze for the duration of its confusion. A creature that passes the saving throw is immune to the Phoenix's chatter for 24 hours.
Flyby. The phoenix doesn’t provoke opportunity attacks when it flies out of an enemy’s reach.
Siege Monster. The phoenix deals double damage to objects and structures.
Actions Multiattack. The phoenix makes two attacks: one with its beak and one with its talons.
Beak. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: (2d8 + 8) piercing damage.
Talons. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: (2d6 + 8) slashing damage.
Pustule Cannons. The Phoenix can, as a action, spray fluid from the pustules that cover its body. All creatures within 20 ft are coated in a substance that causes (2d6) acid damage unless they make a successful saving throw (DC 15 DEX). In addition, targets who take damage from this acid must also make a DC 14 CON saving throw. Those that fail take one level of exhaustion.
Razor Wind.(5-6) The Phoenix flaps its wings, creating an 80 ft line of deadly wind. All targets within the line take (12d6) slashing damage and are knocked 10 ft further away from the Phoenix, falling prone. A Dex saving throw DC 15 halves the damage and negates the movement, as well as preventing the target from being knocked prone.