"The Blade of Winter" Thraximundar

Sneak attack. If your attack roll has advantage your target is surprised or an ally is within 5 ft of the target you deal an extra 14 (4d6) cold damage.

Assassinate. if the target of your attack hasn't made an attack yet or is surprised your attack is an automatic critical hit.

Fey teleportation (1/day). Thraximundar teleports 120 ft. Taking with him any items he is carrying.

Hidden in the Shadows (1/day). Thraximundar uses greater invisibility on himself. Once he does he cant use this ability again until he finishes a long rest.

Trinity of the Winter Sun. If Thraximundar is killed he will revive in 2d6 +1 days, at the next sunrise if both Sedris and Corvax are not killed before that time. On revival Thraximundar retains all items carried upon his death.

 Legendary Resistance (3/day).  If Thraximundar would fail a saving throw he can instead expend a use of his legendary resistance to succeed instead.

Multiattack. When Thraximundar takes the attack action he makes two melee attacks with Winters Grasp and uses wicked strike if able. Or makes 2 ranged attacks.

Winters Grasp. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 9 (1d6 + 6) slashing damage and 7(2d6) cold damage.

Shadow Shot. Ranged Weapon Attack: +11 to hit, reach 60/120 ft., 1 target. Hit: 9 (1d6 + 6) piercing damage and an additional 16 (3d10) necrotic damage.

 Wicked Strike (Recharge 5-6). Thraximundar channels his rage into a powerful attack that undercuts his foes resistances and weakens their physical body. Thraximundar gets advantage on this attack. He targets a creature he can see within 60 ft and teleports behind them, returning to his original position afterwards. They lose all resistances or immunities they may have until the attack finishes. If the attack roll is a critical hit the creature gains vulnerability to all damage dealt by it.

Uncanny Dodge. If Thraximundar would make a saving throw to take half damage instead he takes no damage on a successful save and half damage on a failed save.

Thraximundar has 3 legendary actions and he regains all uses of them at the start of each of his turns

Melee/Ranged Attack (Costs 1 Action). Thraximundar makes a melee or ranged attack.

Wicked strike (Costs 2 Actions). Thraximundar recharges and uses his wicked strike attack.

Fan of knives (Costs 3 Actions). Thraximundar throws a cluster of daggers in a 180 degree arch in front of him. Each creature within 10 feet must succeed on a DC 19 dexterity saving throw or take 25 (10d4) piercing damage.

Blinding powder (Costs 2 Actions). Thraximundar throws a powdered glass substance at a creature within 10 ft. They must succeed on a DC 19 constitution saving throw or become blinded until the end of their next turn.