Deathless. The mage does not require air, food, water or sleep.

Staff. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 5 (1d8 + 1) bludgeoning damage. 

Frostbolt. Ranged Spell Attack: +7 to hit, range 120 ft., 1 target. Hit: 14 (3d8) cold damage and the target's moving speed is reduced by 15 feet until the end of its next turn.

Ice Barrier (recharge 5-6). The Undead Frost Mage surrounds itself with an shield of glimmering frost, and gains 40 temporary hit points.

Frost Nova (recharge 5-6). The Undead Frost Mage emits a wave of freezing cold. All creatures within 10 feet of the mage must succeed on a DC15 Dexterity saving throw or suffer 2d6 cold damage and their movement speed is reduced to zero until the end of their next turn. It may then use its Ice Lance ability against each creature that failed its saving throw.

Ice Lance. Ranged Spell Attack: +7 to hit, range 120 ft., 1 target. Hit: 23 (5d8) cold damage.

Blink. After being hit by a melee attack, the undead mage teleports to an unoccupied space within 15 feet.

Ice Block (once per day). When an enemy makes an attack or casts a spell at the mage, it surrounds itself in a block of ice. The ice has AC12, 100 hit points, immunity to cold, psychic and poison damage and vulnerability to fire and thunder damage. Whilst inside the block the mage is incapacitated but can choose to dispel the ice block at the end of its turn.