Corrupted Frost Knight

Frozen Bulwark.  The knight is immune to effects that enforce movement. 

Iron Grasp. The knight has advantage on athletics checks related to grappling

Enforced Servitude. The Knight's will is inexplicably tied to their master. If the master dies the knight falls to 0 hp and fall unconscious, the knight can be healed but will be revived with 5 levels of exhaustion as the ice still permeates its body. The knight remembers nothing of the time it was corrupted and returns to its previous alignments and personality

Tomb of Ice. (1/day) Upon receiving lethal damage the knight instead enters a tomb of ice. On its next turn the knight heals by the amount of damage the lethal blow caused. The Knights armour is fractured when this effect activates and the Knight has AC 15 when release from the tomb. This has no effect if the lethal damage was Fire damage or damage from a critical hit. If the knight is hit with magical Fire damage it dies immediately when in the tomb, all other damage has no effect during this time. The knight is Deafened and Incapacitated when in the tomb.

Multiattack. The Knight makes three attacks with its glaive. One attack can be Substituted for a Frigid Grip attack each turn.

Glaive. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10+5) slashing damage plus 14 (4d6) Frost Damage

Frigid Grip. The Knight attempts to grab hold of its target and pull it in, covering its body in ice. The knight makes a grapple check against the target. On a success the target has to make a Constitution saving throw with a DC of 15. On a failed save they take 24 (6d6 + 4) cold damage. On a success they take half damage. If the creature is grappled it takes immediate damage and then at the start of every turn it is grappled. Any fire damage received when grappling a creature frees the creature from the grapple.

Frostfall (Recharge 5-6). The knights aura flows into its surroundings and lines surfaces with ice. The knight can surround itself with up to a 30 ft circle or a 60 ft long, 5 ft wide path of ice using itself as the origin. The knight can move double its movement speed in these areas and the affected surfaces act as difficult terrain for non-Frost imbued creatures. Attacks of opportunity made against it are at disadvantage when the Knight is on the ice. Creatures caught in the ice must make a DC 12 Dexterity Saving throw or be restrained until their next turn or until the ice is broken or melted. 

Parry. The knight adds 3 to its AC against one melee attack that would hit it. To do so, the knight must see the attacker and be wielding a melee weapon.