Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.
Multi-Attack: The zombie knight will make two attacks, the first with its Longsword followed by a Shield Bash. Longsword. Melee Weapon Attack: +4 to hit, reach 5ft, one target. Hit: 7 (1d8+2) slashing damage. Shield Bash. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) bludgeoning damage. The target must make a DC 13 Dexterity (Acrobatics) or Strength (Athletics) or suffers one of the following: The target is knocked prone or the target is pushed 5 ft away from the zombie knight.
A terrifying humanoid that has over the centuries rotted away to almost nothing, what is left of its flesh is melded with its armour in some places overgrowing it. It stands 5-6 feet tall and wields a longsword and a Kite Shield of Cold Resistance which grants it and any within 5ft resistance against Cold Damage.
Veil Tower beyond the Wall