Arcturyal, The Demon Lord of Frost

Frozen Body: Arcturyal takes 1d10 extra from fire-based attacks but also heals for 2d10 from any cold-based attacks.

Frost Step: Arcturyal can walk over water at a speed of 20ft per turn.

Spellcasting:
Cantrips (at will): Ray of Frost

Multiattack: The Demon Lord can attack twice, once with melee and once with a ranged attack/spell.

Permafrost Melee Attack: He grabs his target with blistering hands. +6 to hit, Hit: (2d6 + 4) Cold damage 

Glacial Spear. RangedWeapon Attack:  Recharges (5-6). Arcturyal manifests a spear of ice within his hand, throwing it at a target of its choosing +3 to hit, range 30/60 ft., one target. Hit: (4d4 +1 ) Cold damage.

Ray of Frost: Innate Ranged Spell. A frigid beam of blue-white light streaks toward a creature within range. On hit, target takes 1d8 cold damage and its speed is reduced by 10 ft until the start of Arcturyal's next turn. 

Splinter Shard: When Arcturyal takes more than 10 bludgeoning damage from a single melee hit, 2 dmg is reflected back at the target.

Fro-Zone: When an enemy within direct line of sight (3 cardinal directions, not behind) makes a ranged attack from beyond 20 ft, Arcturyal puts up a shield with DC 12 to block the attack. 

Move: The Demon Lord can move again at the end of another creature's turn. Recharges on the start of Arcturyal's next turn.