Arkveld

Armored Bonescale. The Arkveld is immune to critical hits and effects from critical hits.

Corpse Feeder. If the Arkveld reduces a creature to 0 hit points it regains 20 hit points and its attacks deal an additional 22 (4d8) necrotic damage until the end of its next turn.

Energy Gorger. When the Arkveld scores a critical hit on another creature the damage it deals reduces the creature's maximum hit points by an amount equal to half the damage done, additionally the Arkveld recharges their Energy Unleashed ability.

Insatiable Guardian (Mythic trait; Recharges after a Short or Long Rest). If the Arkveld is reduced to 0 hit points, it doesn't die or fall unconscious. Instead, it's bones shatter and it regains 150 hit points but loses its resistance to slashing and piercing damage and its Armored Bonsescale trait.

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

Multiattack. The dragon can use its Frightful Presence. It then makes four attacks: one with its bite, two with its bone lashes and one with its tail.

Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 16 (2d8 + 7) piercing damage.

Bone lashes. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) slashing damage plus 9 (2d8) necrotic damage.

Tail. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 20 (2d12 + 7) bludgeoning damage.

Frightful Presence. Each creature of the Arkveld's choice that is within 120 feet of the creature and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Arkveld's Frightful Presence for the next 24 hours.

Energy Unleashed (Recharge 5–6). The Arkveld releases energy from its bone lashes in a 30-­foot cone. Each creature in the cone must make a DC 18 Dexterity saving throw, taking 54 (12d8) necrotic damage on a failed save, or half as much damage on a successful one.

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

Reposition. The Arkveld moves up to half of its movement speed without provoking opportunity attacks.

Tail Attack. The Arkveld makes a tail attack.

Lash Drain (Costs 2 Actions). The Arkveld makes two Bone lashes attacks against one creature. Afterwards the Arkveld gains temporary hit points equal to half the total damage done, additionally the Arkveld regains a legendary action for each attack that hits. This legendary action may be used only once per round.

If Arkveld’s mythic trait is active, it can use the options below as legendary actions for 1 hour after using Insatiable Guardian.

Lash Out. The Arkveld makes two attacks, one with its tail and one with its bite.

Sky Reaver (Costs 2 Actions). The Arkveld flies up to its movement speed without provoking opportunity attacks and makes up to three attacks with its bone lashes or uses its energy unleashed.