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.

Multiattack. The zombie knight makes two mêlee attacks.

Diseased Greatsword. Mêlee weapom attack: +5 to hit, reach 5ft, one target. Hit: 10 (2d6 +3) slashing damage. After a creature is hit by this greatsword the target has to make a constitution saving throw (DC 15). On a failed save the target is infected with Sight rot (DMG 257). If the target is already infected nothing else happens.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage.

Heavy crossbow. Ranged weapon attack: +0 to hit, range 100/400 ft., one target. Hit: 5 (1d10) piercing damage.