Arthas Menethil, the Lich King

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

Magic Resistance. Arthas has advantage on saving throws against spells and other magical effects.

Helmet of Domination. The helmet of the Lich King gives Arthas direct command of all nearby undead, allowing them to execute complex battle tactics.

Heart of Ice. Arthas' heart is encased in a globe of ice on top of the Frozen Throne. At the start of Arthas' turn, he regains 20 hit points unless the Heart is destroyed. It is a small object with  an AC of 16 and 30 HP.

Turn Immunity Aura. Undead within 100ft. of Arthas have immunity to features that turn undead.

Frozen tempest. Arthas radiates a biting cold atmosphere as an aura around himself. At the start of Arthas' turn, any creature other than Arthas and his undeads within 40 ft. of him take 10 (3d6) cold damage. In addition, any melee attacker that hits Arthas for the first time in any turn takes 10 cold damage.

Eye of the storm. Strong, icy winds rage in a sphere with a radius of 15 ft. around Arthas. Any creature except Arthas and his undeads that starts its turn inside the sphere has its speed reduced by 10 ft. until it leaves the area of effect. This effect cannot reduce a creature's speed to less than 20 ft.

Eldritch Smite.  Once per turn when Arthas hits a creature with Frostmourne, he can expend a spell slot to deal an extra 4 (1d8) cold damage to the target, plus another 1d8 per level of the spell slot, and he can knock the target prone if it is Huge or smaller.

Spellcasting. Arthas is a 10th-level spellcaster. His spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). He has the following spells prepared:

1st level (4 slots): armor of agathys, shield, wrathful smite

2nd level (3 slots): hold person, spiritual weapon 

3rd level (3 slots): dispel magic, spirit guardians 

4th level (3 slots): banishment, shadow of moil

5th level (2 slots): cone of cold

Multiattack. Arthas makes three longsword attacks with Frostmourne.

Frostmourne. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 6) slashing damage, plus 13 (3d8) necrotic damage.

Death Coil (Recharge 5-6). Ranged Spell Attack: Arthas unleashes a bolt of pure undeath: +10 to hit; range 30 ft., Hit: 45 (10d8) necrotic damage. On a successful hit, the target must make a DC 18 Constitution saving throw. If the target fails this save, Arthas is healed for the full damage dealt by this ability, and the target is frightened of him until the end of its next turn. If the targets succedes, Arthas is healed for half the damage dealt and the target does not suffer any of the additional effects.

Conjure Undead (1/Day). Arthas conjures Undead creatures whose combined average hit points don’t exceed 150. These creatures magically rise up from the ground or otherwise form in unoccupied spaces that Arthas can see within 150 feet of him and obey his commands until they are destroyed or until he dismisses them as an action. As a bonus action, Arthas can destroy any number of zombies he summoned. He is healed 15 HP for each undead destoryed this way.

Parry. Arthas adds 6 to its AC against one melee attack that would hit it. To do so, Arthas must see the attacker and be wielding a melee weapon.

Arthas can take 4 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. Arthas regains spent legendary actions at the start of his turn.

Frostmourne. Arthas makes a single melee attack whit Frostmourne

Death grip (Costs 1,2 or 3 Actions). Arthas raises his hand towards a creature he can see within 90 ft. of him. That creature must make a Strenght saving throw or be pulled to the closest available space in front of Arthas, as tendrils made of shadow bind it and bring it close to Arthas.
If the creature is within 30 ft. of Arthas this costs 1 action and the DC is 17.
If the creature is between 30 ft. to 60 ft. from Arthas this costs 2 actions and the DC is 16.
If the creature is between 60 ft. to 90 ft. from Arthas this costs 3 actions and the DC is 15.

Quake. Arthas stabs Frostmourne into the ground, causing a quake in a circle with a 50ft. radius  around him. All within that radius must make a DC 16 Dexterity saving throw or be knocked prone.

Harvest Souls (costs 3 actions).  all foes within 40 ft. of Arthas must make a DC 17 Constitution saving throw or take 15 (3d10) necrotic damage. Arthas is healed by half of the total damage dealt by this ability.