Arkveld

Corpse Feeder. If the Arkveld reduces a creature to 0 hit points it regains hit points equal to half of the damage dealt on the attack that killed the creature.

Fear Gorger. When a creature is Frightened by the Arkveld, the creature’s energy is sapped. The creature will take 8 (2d8) Psychic damage as well as gain a level of exhaustion for every turn it is frightened within 60 feet of the Arkveld. The Arkveld gains hit points equal to half of the damage taken by a frightened target, rounded down. On removal of the Frightened condition, all levels of exhaustion are removed from the creature. 

Chain Splitting. When the Arkveld Enrages, its two chains separate apart, granting it the ability to perform four Chainlash attacks on its multiattack, as well as the ability to perform its Energy Release attack. Each chain has the following stats: AC 16; 40 hit points; Vulnerability to Force damage, Resistance to cold, fire, lightning, and acid damage, Immunity to psychic damage. While having the chains active, the Arkveld adds 6 (2d6) Draconic damage to its Chainlash attacks. Upon a chain being reduced to 0 hit points, the Arkveld is stunned for 1 turn and loses the ability to perform 4 Chainlash attacks as a multiattack. 

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

Multiattack. The Arkveld uses its Frightful Presence. It then can perform two Chainlash attacks. Any one of the Chainlash attacks can be substituted with a Tail attack. 

Frightful Presence. Every creature that is within 600 feet of Arkveld must succeed on a DC 18 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 Arkveld’s Frightening Presence for the next 24 hours.

Chainlash. Melee Attack Roll: +10, reach 25 ft. Hit: 11 (2d6 + 4) Slashing damage. If the target is a Huge or smaller creature, it must make a DC-14 Strength or Dexterity saving throw, becoming Grappled by the chain and taking an extra 14 (4d6+2) Bludgeoning damage (Escape DC-19 Strength). When a target is grappled they cannot be targeted by a Chainlash attack. 

Tail. Melee Attack Roll: +13, reach 20 ft. Hit: 28 (3d12 + 10) Bludgeoning damage. The target must succeed a DC-18 Strength or Constitution saving throw or be knocked Prone.

Fissure. The Arkveld stabs two of itsits Chainlashes into the ground, causing a burst of dragon energy in a 20 foot cone centering at a point within 10 feet of the Arkveld. All creatures within the area of the cone must make a DC-17 Dexterity saving throw. On a failed save, the target takes 42 (5d12 + 12) Draconic damage and is knocked Prone. On a successful save the target takes half damage and is not knocked prone. The terrain within the area is turned into difficult terrain.

Energy Release. The Arkveld stabs every single one of its Chainlashes into the ground, causing the ground to shatter as an eruption of dragon energy bursts out in a 30 foot radius centering at a point within 25 feet of the Arkveld. All creatures within the area of the cone must make a DC-17 Dexterity saving throw. On a failed save, the target takes 42 (5d12 + 12) Draconic damage and is knocked Prone. On a successful save the target takes half damage and is not knocked prone. All area within the radius of the attack is turned into difficult terrain. 

Unnerving Gaze. Trigger: A creature the Arkveld can see starts its turn within 25 feet of the Arkveld and can see it. Response—Wisdom Saving Throw: DC 18, the triggering creature. Failure: The target has the Frightened condition for one hour or until it is otherwise removed. Success: The target is immune to this Arkveld’s Unnerving Gaze for 24 hours. If a creature moves 60 feet away from the Arkveld then it loses the Frightened condition but it does not become immune to the Arkveld’s Unnerving Gaze.

The Arkveld 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 Arkveld 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 absorbs the energy of a grappled creature. The creature can make 2 Escape saves before this happens, and upon failure the creature takes 46 (5d10+21) Necrotic damage and falls Prone onto the ground, not taking fall damage. Afterwards the Arkveld gains temporary hit points equal to half the total damage done.

Rend (Costs 3 Actions). The Arkveld moves up to their flight speed and performs Three Chainlash attacks on the same target. A creature cannot be affected by the Grappling part of the Chainlash attack via this action.