Burnt Ultra Great-cleaver. Yhorm's great cleaver's attacks are magical. When Yhorm hits with a weapon attack it does an extra 2d8 fire damage.

Legendary Resistance. If Yhorm fails a saving throw he can choose to succeed instead. Half damage on a success.

Berserker smash. Grabs the second hand notch on the other side of the haft of the great cleaver and brings his pain and anger to wrought, smashing down in front of him 4 times moving 20ft forward. Dexterity saving throw of 14 to dodge. Can only hit one enemy once. 1d12 + 6 on hit. 

Great Cleaver. Melee Weapon Attack: +0 to hit, reach 10 ft., 1 target. Hit: 5 (2d8 + 8) [damage type] damage. 

Defiance of Undeath. Yhorm channels is flame to his leg and stomps on the ground exploding with fire, a shock wave shoots out from the ground as he's stomping that is followed by curtains of flame. Radius of 15 feet around Yhorm. Enemies within the radius must roll a Strength saving throw DC10, and a dexterity saving throw DC10. On the strength saving throw, if a failure the target is blown backwards to prone 10 feet and take 1d6+3 force damage, on a success they are not thrown backwards, and do not take damage. On the dexterity saving throw, a failure means they take 1d8+4 fire damage, success means half damage. 

The undead legion (Costs 2 Actions). Yhorm emits a terrible roar, seemingly calling into the abyss. His people, long lost to the ages answer and come to him in his time of need, as he did for them. Spawns skeletons in the room around him.