Sustained by the Elements. As long as Ymir has ice to absorb within 30 feet and his crown is intact, he regains 50 HP at the start of each of his turns, as his chains reach for more ice to incorporate into his form.

The Missing Gem. The gem within Ymir's crown can be pried off with a DC15 STR check while he is incapacitated, or a DC25 STR check while his is not. Doing so disables his regeneration

Multiattack. Ymir uses his Ice Burst ability and makes two chain or hurl attacks. 

Ice Burst. Ymir chooses three unoccupied spaces within 60 feet of him that he can see. Create an ice mine token in each space, then each creature within 5 feet of one or more of those spaces must make a DC18 DEX save or suffer 3d10 piercing damage, or half damage on a success.

Chain. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 25 (3d12 + 6) slashing damage.

Hurl. Ranged Weapon Attack: +9 to hit, range 60/240 ft., one target. Hit: 28 (4d10 + 6) bludgeoning damage.

Smash Ice. Ymir smashes the ice in front of him in a 60 foot cone. Any creatures in the cone AND within 10 feet of him must make a DC18 DEX save or suffer 6d12+6 damage.

Legendary Actions 3/round

Ice Burst: Ymir uses his Ice Burst ability on a single target

Detonate Ice Mines (Costs 2 actions): Each creature must make a DC18 DEX save for each mine they are within 10 feet of, suffering 6d6 cold damage for each failure or half as much for each success.