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.

Herd movement. The Zombie herd has a 15 foot diameter, and is generally a circle, but can take other shapes. It can share a space with other creatures and move around obstacles. It has a reach of 5 ft from any edge of its space and can hit targets within. The herd will move together, but can split off into smaller groups, reducing its size and health accordingly.

Overwhelming Stench. If a creature starts its turn within the herd, it must make a constitution saving throw (DC 8) or become overwhelmed with the stench of decaying flesh around it. On a failed save the creature becomes incapacitated until its next turn. On a successful one it is unaffected. A creature that attempts to cover or block the smell has advantage on this roll. A creature with no sense of smell is immune to this effect.

Multi-attack. The Zombie Herd can make 4 attacks on its turn, but can only hit one target twice.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

Bite. Melee Attack: +2 to hit, reach 5 ft., one target. Hit: 3 (1D4 + 1) Piercing damage.

Death's Grasp. A zombie from the herd can try to restrain a target and attempt to bite it. The target must succeed on a contesting Strength check against the individual zombie or be restrained until the start of its next turn, when it can make the check again. While restrained the target can still be attacked by other member's of the herd.