Avatar of Surtur

Fire Absorption. Whenever Surtur is subjected to fire damage, he takes no damage and instead regains a number of hit points equal to the fire damage dealt.

Fire Aura. At the start of each of Surtur’s turns, each creature within 5 feet of him takes 11 (2d10) fire damage, and flammable objects in the aura that aren’t being worn or carried ignite. A creature that touches Surtur or hits him with a melee attack while within 5 feet of him takes 11 (2d10) fire damage.

Innate Spellcasting. Surtur’s innate spellcasting ability is Charisma (spell save DC 23). He can innately cast the following spells, requiring no material components:

At will: aganazzar’s scorcher (3rd-level version)

3/day each: burning hands (5th-level version), wall of fire

1/day each: delayed blast fireball, fire storm, incendiary cloud, meteor swarm

Legendary Resistance (4/day). If Surtur fails a saving throw, he can choose to succeed instead.

Magic Resistance. Surtur has advantage on saving throws against spells and other magical effects.

Magic Weapons. Surtur’s weapon attacks are magical.

Mark of the Krig Rune. Surtur’s allies are immune to being frightened while within 20 feet of him.

Regeneration. Surtur regains 20 hit points at the start of his turn. If he takes cold damage, this trait doesn't function at the start of his next turn. Surtur dies only if he starts his turn with 0 hit points and doesn't regenerate.

Siege Monster. Surtur deals double damage to objects and structures.

Multiattack. Surtur makes two greatsword attacks.

Greatsword. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 36 (8d6 + 8) slashing damage plus 28 (8d6) 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 11 (2d10) fire damage at the start of each of its turns.

Rock. Ranged Weapon Attack: +16 to hit, range 150/600 ft., one target. Hit: 34 (4d12 + 8) bludgeoning damage and if the target is Large or smaller, it must succeed on a DC 24 Strength saving throw or be knocked prone and restrained until it escapes out from under the rock. Any creature can use its action to make a DC 18 Strength (Athletics) check to free itself or another creature under the rock, ending the effect on a success. The rock is an object with 15 AC, 30 hit points, vulnerability to thunder damage, resistance to piercing and slashing from nonmagical weapons not made of adamantine, and immunity to necrotic, poison, and psychic damage.

Armor of Fire (1/day). Surtur gains 42 temporary hit points which last for up to one hour. While any of these hit points remain, he has resistance to cold damage.

Surtur 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. Surtur regains spent legendary actions at the start of his turn.

Hurl Missile. Surtur makes a rock attack.

Move. Surtur moves up to half his speed.

Fling (Costs 2 Actions). Surtur makes an attack with his greatsword. On a hit, he pushes the target up to 60 feet away from him horizontally or vertically in a direction of his choice

Raging Charge (Costs 2 Actions). Surtur moves up to 30 feet in a straight line and can move through the space of any creature smaller than Huge. The first time he enters a creature's space during this move, he can choose to make a greatsword attack against that creature. Ifthe attack hits, the target must also succeed on a DC 24 Strength saving throw or be pushed ahead of Surtur for the rest of this move. If a creature fails the save by 5 or more, it is also knocked prone and takes 22 (4d6 + 8) bludgeoning damage, or 36 (8d6 + 8) bludgeoning damage if it was already [/condition]prone[/condition].

Spell (Costs 2 Actions). Surtur casts a spell using his Innate Spellcasting trait.