Arkveld

Absorb Elementals. If Arkveld is not in Full Power mode, attacks of type fire, cold, acid, lightning, or force are absorbed as energy, granting Arkveld 1 Full Power  Charge instead of dealing damage to its HP (force damage grants 2 Full Power Charges).

Full Power. When Arkveld accumulates 6 charges, it enters Full Power Mode for 3 rounds. Upon entering this mode, Arkveld gains 50 temporary hit points, all of its abilities with a Recharge condition immediately recharge, and all negative conditions currently affecting it are removed (the recharge and condition removal occur only once when the mode is activated).Additionally, while in Full Power Mode, Arkveld cannot gain additional Full Power Charges, all of its attacks deal an additional 6 (2d6) force damage, and its attack rolls cannot be made with disadvantage.

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

Multiattack. The Arkveld makes three melee attacks: two Chainblade Attack, one Double Chainblade Attack or a Chainblade Dive Attack.

Chainblade Attack. Melee Weapon Attack Range: 15 feet one target. +12 to hit Hit: The target takes 16 (3d6 + 7) bludgeoning damage and 6 (2d6) Force damage. If the target is a Huge or smaller creature, it is grappled.

Double Chainblade Attack. Melee Weapon Attack Range: 20 feet one target. +12 to hit Hit: The target takes 19 (3d8 + 7) bludgeoning damage and 6 (2d6) Force damage. If the target is a Huge or smaller creature, it has the Prone condition.

Chainblade Dive (Recharge 3). The Arkveld flies above the ground, then lunges forward in a straight line for 40 feet, dragging its chainblade along the ground without provoking opportunity attacks. Each creature in its path must make a DC 18 Dexterity saving throw. On a failed save: The creature takes 13 (2d6 + 7) bludgeoning damage and 6 (2d6) Force damage. and is knocked prone. On a successful save: The creature takes half damage and is not knocked prone.

Red Lightning Blast (Recharge 4). Arkveld gathers energy into its chainblade and releases it in a 60- feet-long, 15- feet-wide line in front of it. Each creature in the area must make a DC 18 Dexterity saving throw. On a failed save: The creature takes 51 (17d6) force damage, is knocked prone and is pushed 5 feet. On a successful save: The creature takes half damage and is pushed 5 feet.

Energy Drain. While Arkveld has a creature grappled, it can use a bonus action to drain that creature’s life force. This action cannot be used while Arkveld is in Full Power Mode. Each grappled creature must make a DC 18 Constitution saving throw. On a failed save: The creature’s Constitution score is reduced by 1d6, and Arkveld gains 1 Full Power Charge for each creature drained. This reduction lasts until the creature finishes a long rest. If this effect reduces a creature’s Constitution score to 0, the creature falls unconscious. When it regains consciousness, its Constitution score becomes 1, and it gains one level of exhaustion if it fell unconscious due to this effect.

Chainblade Smash. While Arkveld has a creature grappled, it can use a bonus action to make a melee weapon attack against that creature. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 16 (3d6 + 7) bludgeoning damage plus 6 (2d6) force damage. The grapple then ends, and the target is pushed 10 feet away from Arkveld.

Jump Maneuver (Recharge 5). Arkveld uses its chainblade to pull itself away from an attack, moving 20 feet without provoking opportunity attacks. This maneuver cannot be used against magic missile, spells that require a saving throw (such as sacred flame), or aura-based spells (such as spirit guardians). 

Dragon Roar (Recharge 6). When Arkveld takes damage, it lets out a terrifying roar. Every creature within 60 feet that hears the roar must make a Wisdom saving throw (DC 18). On a failed save, the creature is frightened until the end of its next turn.

Available: At the end of another player's turn (before the next turn begins). When one is used, 1 point will be deducted from the 3 points available in that round. Points refresh at the beginning of each Arkveld's round.

Chainblade Blast (1/round). Arkveld gathers energy into its chainblade and releases it in a 20-foot cone. Each creature in the area must make a DC 18 Dexterity saving throw. On a failed save, a creature takes 12 (4d6) force damage and is knocked prone. On a successful save, a creature takes half as much damage.

Tail Thrust (1/round). Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 16 (3d6 + 7) bludgeoning damage plus 6 (2d6) force damage, and is pushed 5 feet away from Arkveld

Tail Spin Maneuver (1/round). Arkveld spins, striking with its tail in all directions. Each creature within 10 feet of it must make a DC 18 Dexterity saving throw.On a failed save: A creature takes 13 (2d6 + 7) bludgeoning damage plus 6 (2d6) force damage, and is pushed 5 feet away from Arkveld. On a successful save: A creature takes half as much damage and isn't pushed.

Chainblade Strike (1/round). Arkveld moves up to 20 feet in a straight line. This movement doesn't provoke Opportunity Attacks. While moving, Arkveld drags its Chainblade in a 15-foot-wide path. Each creature in that area must make a DC 18 Strength saving throw.On a failed save: A creature takes 13 (2d6 + 7) bludgeoning damage plus 6 (2d6) force damage and is knocked prone.On a successful save: A creature takes half as much damage and isn't knocked prone.

Chainblade Charge (1/round). Arkveld moves up to 40 feet toward a target without provoking Opportunity Attacks. At the end of this movement, it makes a melee weapon attack with its chainblade.Melee Weapon Attack: +12 to hit, reach 5 ft., one target.Hit: 19 (3d8 + 7) bludgeoning damage plus 6 (2d6) force damage. If the target is a Huge or smaller creature, it is knocked prone and pushed 10 feet away from Arkveld.