Vakthund, Watchdog of the Old Gods (Chaos Form)

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

Omniscient Defender. Vakthund knows the location of any creature within its territory, as long as that creature has a scent.

Undying Loyalty. Vakthund cannot be permanently killed within its territory, even if its body is entirely obliterated. Upon its death, it regenerates in a new form at the center of its domain in 4d6 hours. At this time, its howl can be heard by all within its territory. Effects that move it to a different Plane or otherwise forcibly remove it from its territory in a non-physical manner have no effect.

Elemental Affinity (Chaos). Vakthund's body contains the full power of all its elemental forms, unleashed at their full potential. Its bite attacks count as magical for the purposes of damage calculation, and deal an additional 3d10 lightning damage, 3d10 cold damage, and 3d10 fire damage (included in attack, one per each hit in multiattack). It has immunity to all thunder and lightning damage, and resistance to its other two forms' elemental damage. This affinity changes with its form.

Multiple Heads. Vakthund has doubled advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. While it sleeps, at least one head is awake. It can take as many reaction attacks as it has heads (3).

Chaos Shroud. Vakthund is constantly shrouded in a ring of powerful winds, lighting, ice, and flame. All creatures that end their turn within 15 ft. of Vakthund take 8 (2d8) lighting damage, 8 (2d8) cold damage, and 8 (2d8) fire damage, and must make a DC 20 Strength saving throw or be thrown backwards 30 ft. and prone. 

While this whirlwind persists, all ranged attacks against Vakthund are taken at disadvantage, and any creature that attempts to move closer than 15 ft. to Vakthund must make the above Strength DC or be blown backwards. This effect ends if Vakthund is incapacitated, and resumes upon the end of that effect.

Multiattack. Vakthund can use its Baneful Howl, use one of its breath attacks and make three bite attacks. On its turn, Vakthund can either make one attempt to recharge its Chaos Breath, or two attempts to recharge its lesser breath attacks.

Baneful Howl. All creatures within 300ft. of Vakthund must make a DC 24 Wisdom saving throw, becoming frightened on a failed save. The affected creature(s) can make the saving throw at the end of their turn, ending the condition on a successful save. Upon a successful saving throw, the creature(s) are immune to Vakthund's Baneful Howl for 24 hours.

Bite. Melee Weapon Attack: +18 to hit, reach 5 ft., one target. Hit: 10 (3d8 + 10) piercing damage and 15 (3d10) lightning/cold/fire damage.

Chaos Breath (Recharge 6).  Vakthund's three heads combine all of its breath attacks into a massive blast of energy in a 90 foot cone. All creatures within the area of effect must make a DC 24 Dexterity saving throw or take 40 (10d8) lightning damage, 40 (10d8) cold damage, and 40 (10d8) fire damage. Affected creatures must make a DC 22 Constitution saving throw. Upon failure, a d4 is rolled for each creature. On a 1 the creature is frostbitten (see Freezing Breath), on a 2 or 3 the creature is paralyzed until the end of Vakthund's next turn. On a 4, the creature takes an additional 5d8 damage of each damage type. Creatures affected by the statuses can make a saving throw at the end of their turn, ending the effects on a success.

Magmatic Breath (Recharge 5-6).  Vakthund's central head vents a glut of burning magma in a 10x15 ft. area in front of it. All creatures within the area of effect must make a DC 24 Dexterity saving throw or be restrained in the hot magma and take 96 (24d8) fire damage (creatures are not restrained on a successful save and take half damage). Restrained creatures must make a DC 18 Strength check to escape. Any creature that initially enters the area or ends its turn in this area receives 3d10 additional fire damage. The affected area burns in this fashion for three rounds. The area is difficult terrain for the duration.

Lightning Breath (Recharge 5-6).  Vakthund's central head fires a bolt of lightning in a 10 ft. wide line that is 120 ft. long. All creatures within the area of effect must make a DC 22 Dexterity saving throw or take 80 (20d8) lightning damage. Affected creatures must make a DC 20 Constitution saving throw or be paralyzed until the end of Vakthund's next turn. Paralyzed creatures can repeat the saving throw at the end of their turn, ending the effect on a success.

 Freezing Breath (Recharge 5-6).  Vakthund's central head exhales a blast of intense cold in a 60 ft. cone. All creatures within the area of effect must make a DC 24 Constitution saving throw or take 80 (20d8) cold damage and become frostbitten, half damage on a successful save.

While frostbitten, creatures have their movement speed halved, cannot take reactions, and can only take an Action or Bonus Action on their turn. The affected creature(s) can repeat the saving throw at the end of their turn, ending the effect on a success. Creatures sustain 1d8 cold damage upon each successive failed save, and upon failing a third save become frozen. The frozen status effect functions similarly to the petrified status, but can be ended by the creature taking any form of fire damage, or otherwise being warmed to a high degree.

Vakthund 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. Vakthund regains spent legendary actions at the start of its turn.

Call to the Gods (Costs 2 Actions). Vakthund summons a barrage of lightning, fire, and ice upon the area within 120 ft. of it. All creatures within this radius must make a DC 24 Dexterity saving throw or take 16 (4d8) lightning damage, 16 (4d8) fire damage, and 16 (4d8) cold damage, half on a successful save. All creatures within this radius must additionally make a DC 24 Constitution saving throw or take 32 (8d8) thunder damage and be deafened. Creatures within 30 ft. of Vakthund take doubled damage.

Deafened creatures can repeat the saving throw at the end of their turns, ending the effect on a success.

Heavenly Gale. Vakthund sends out a gust of wind in a 60 ft. wide line in a direction it chooses.  All creatures within 30 ft. of this wall of wind must make a DC 24 Strength saving throw or be blown backwards 30 ft and prone, taking 4d8 bludgeoning damage.

Chaos Rush. Vakthund becomes wreathed in all three forms of energy and can move up to its movement speed and making a bite attack against a target. That creature takes an additional 3d10 cold, 3d10 fire, and 3d10 lightning damage upon a successful hit (6d10 of one damage type when combined with the original attack). Any creature within 15 ft. of Vakthund as it passes must make a DC 22 Dexterity saving throw, or take 3d10 damage of one of the types above (half on a successful save).