Black ice golem

Cold aura. Each creature that moves within 20ft or starts their turn within 20ft of the black ice golem takes 6 (2d6) cold damage. 

Freezing body. A creature that touches the black ice golem or hits it with a melee attack while within 5 feet of it takes 3 (1d6) cold damage.

Ice Walk. The black ice golem can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Specific magic immunity. The black ice golem is immune to all cold and necromancy based spells and has advantage on saving throws against all types of magic except fire magic. 

Magic Weapons. The golem's weapon attacks are magical.

Multi attack. The black ice golem makes two slam attacks. 

Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 19 (3d8 + 7) bludgeoning damage plus 6 (2d6) cold damage and the target's maximum hit points are reduced by the amount of cold damage taken. The black ice golem also heals 1/3rd of the hit points the target was reduced by. 

Ice shards (Recharges on 5/6). The black ice golem blasts out shards of ice in a 10ft radius around itself. Each creature within range must make a DC 17 Constitution saving throw or take 9 (3d6) cold damage and be blinded for 30 seconds. A creature who fails this save can remake the save at the end of their turn to rid themselves of the blindness.