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.

Retaliation. When a creature hits the zombie with an attack, it may make one Slam attack (no reaction). If this attack hits, the target makes a DC 14 Strength Saving throw. On a failure, the target is Grappled (escape DC 14). The zombie can only grapple one creature at a time.

Brutish Grappler. This creature has advantage to hit any creature that is grappled by it.

Longsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage.

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

Necrotic Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target that is grappled by the zombie. Hit: 6 (1d6+3) piercing damage, plus 10 (3d6) necrotic damage. The target's maximum hit points are reduced by the amount of necrotic damage taken. This gives the target a Festering Wound (see below).

Festering Wound. Condition: A creature with a festering wound is grievously wounded. Each nightfall, the creature takes 18 (4d8) necrotic damage and gains one level of exhaustion. The creature's maximum hit points are reduced by the amount of necrotic damage taken. If a creature has 0 maximum hit points, it immediately dies. If a living creature dies while it has a Festering Wound, it becomes a Zombie. A zombie created in this way attacks the nearest living creature. A Festering Wound can be removed by a casting of Greater Restoration, or by receiving magical healing greater than or equal to 15 hit points within the span of 1 turn. By using one use of a healer's kit or casting Lesser Restoration on an infected creature, the infection is held back; the next nightfall has no negative effect on the creature.