Valhir, the Thousand Pierced Bear

Legendary Resistance (5/Day). If Valhir fails a saving throw, it can choose to succeed instead.

Unrelenting Storm. If a creature touches or hits Valhir with a melee attack, they take 14 (4d6) lightning damage. Additionally, all lightning and thunder damage dealt by Valhir ignores resistance and treats immunity as if it were resistance.

Relentless Lightning. When Valhir lands five melee attacks in a round, the lightning coursing through his blackened fur arcs outward, and Valhir can cast Chain Lightning without any verbal, semantic, or material components on each creature of his choice within 60 feet of him. Valhir's Spell Save DC is 25.

The Final Challenge (Once Per Short Rest). When Valhir is reduced to 0 hit points, he lets loose an unparalleled blast of destructive lightning as a final challenge to his enemies. Each creature within 300 feet of Valhir must succeed on a DC 25 Constitution saving throw or take 55 (10d10) lightning damage, plus 55 (10d10) thunder damage and be knocked prone. Valhir regains 50 hit points for each creature that reduced to 0 hit points by this ability. On a success, the creature takes half as much damage and is not knocked prone. Objects, structures, and terrain automatically fail this save.

Siege Monster. Valhir deals double damage to objects and structures

Multiattack. Valhir, the Thousand Pierced Bear makes five melee attacks, only one of which can be Kjever, and uses Lyn once if available. Valhir can replace any of his attacks with a grapple attempt.

Klør. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 24 (4d6 + 10) slashing damage, plus 7 (2d6) lightning damage. 

Neve. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 24 (4d6 + 10) [bludgeoning] damage. If the attack lands, the target is thrown 20ft. in a direction of Valhir's choice. If the target moves at least 5 feet and impacts a creature, object, or piece of terrain, they take an additional 18 (4d8) bludgeoning damage. 

Kjever. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 31 (6d6 + 10) piercing damage, plus 14 (4d6) lighting damage. If the attack lands, Valhir cripples the target with his sky-shattering jaws, reducing their movement speed to a maximum of 10 feet until they are magically healed. 

Lyn (Recharge 4-6). Valhir summons a titanic blast of purple lightning from above, which impacts at a point he can see within 300 ft. of Valhir. Each creature within 30 ft. of that point must succeed on a DC 25 Constitution saving throw or be judged weak and take 60 (10d8 + 15) lightning damage and be knocked prone.

Storm of Vengeance. Valhir can cast Storm of Vengeance as a bonus action at will without requiring material or semantic components or concentration. 

Maul the Weak. When a creature within 120 ft. of Valhir misses an attack on Valhir, the Thousand Pierced Bear can use his reaction to make a Kjever attack against that creature should they be within reach.

Valhir can take 5 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. Valhir regains spent legendary actions at the start of its turn.

Strike (Costs 1 Action). Valhir makes one Klør or Neve attack against a target in range.

Crush (Costs 2 Actions). Valhir crushes a target he is currently grappling with his monstrous, clawed hands. The target takes 36 (8d8) bludgeoning damage.

Charge (Costs 3 Actions). Valhir moves up to his full movement speed, making one Klør or Neve attack against each creature within his melee range during the charge.

Gathering Storm (Costs 5 Actions). Valhir calls to the sky, channeling the storm into his grasp before slamming it into the earth. The ground and air in a 60 ft. radius around Valhir then becomes electrified. This area is considered difficult terrain and creatures and objects take 14 (3d8) lightning damage for every 5 feet they move in this space.