Archguardian

Aura of Serenity. The archguardian exudes an aura that brings injured foes to sleep within 30ft of it. When a hostile creature with less than half of its maximum hit points starts its turn in this aura, it must succeed on a DC17 Constitution saving throw or take one level of exhaustion. If it fails the save a second time at the start of the next turn, its movement to all its speed is reduced by 20ft and it cannot take multiple actions. On a third continuous failed save, it falls asleep becoming unconscious, for one minute. A creature can shake a sleeping creature awake as an action on its turn if it is within 5ft of it and as long as this creature isn't in the archguardian's aura anymore

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

Limited Magic Immunity. The archguardian can't be affected or detected by spells of 5th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

Magic Weapons. The archguardian's weapon attacks are magical.

Unbreakable. The archguardian has advantage on saving throws against being incapacitated.

Multiattack. The archguardian makes three attacks: two with its claws and one with is bite, tail or horns.

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 25 (4d8 + 7) piercing damage. If the target is a creature, it must succeed on a DC17 Constitution saving throw or take 17(5d6) psychic damage as well.

Claw. Melee Weapon Attack: +12 to hit, reach 15ft., one target. Hit: 20 (3d8 +7) slashing damage.

Horn. Melee Weapon Attack: +12 to hit, reach 10ft., one target. Hit: 23 (3d10 +7) piercing damage. If the target is a Medium or small creature, it is also impaled. While impaled, it is restrained (Escape DC17).

Stringer Tail. Melee Weapon Attack: +12 to hit, reach 20ft., one target. Hit: 20 (3d8 +7) piercing damage plus (5d10) poison damage.

Invisibility. The archguardian magically turns invisible until it attacks or cast a spell, or until its concentration ends (as if concentrating on a spell). Any equipment the archguardian wears or carries is invisible with it.

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

Claw Attack (Costs 1 Action). The archguardian makes a claw attack.

Teleport (Costs 1 Action). The archguardian magically teleports, along with any equipment it is wearing or carrying, up to 12o feet to an unoccupied space it can see.

Wing Attack  (Costs 2 Action). The archguardian beats its wings. Each creature within 25 ft. of the archguardian must succeed on a DC17 Dexterity saving throw or take 20 (3d8+7) bludgeoning damage and be knocked prone. The archguardian can then fly up to half its flying speed.