Undead Fortitude. If damage reduces the Zombie Dreadhorde 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 Dreadhorde drops to 1 hit point instead.

Tightly Packed. The Zombie Dreadhorde has disadvantage on saving throws against spells, effects, and conditions that encompass at least half of the spaces occupied by it.

Horde. The horde can occupy another creature's space and vice versa, and the horde can move through any opening large enough for a Medium creature. Enemies of the horde treat any spaces occupied by the horde as difficult terrain. The horde can't regain hit points or gain temporary hit points. The horde is unaffected by spells, effects, and conditions except damage that do not apply to at least half of the squares that the horde occupies.

Stench. The Zombie Dreadhorde exudes a horrid stench in a 20ft emanation. Any creature that is not undead that enters the emanation for the first time on a turn or that starts its turn there must succeed on a DC 14 Constitution saving throw or be poisoned until the start of the its next turn. On a successful saving throw, the creature is immune to the Zombie Dreadhorde's Stench for 24 hours.

Multiattack. The Zombie Dreadhorde makes eight melee attacks, only half of which can be bite attacks. If the Zombie Dreadhorde is bloodied, it makes 4 melee attacks, only half of which can be bite attacks.

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage. If the Zombie Dreadhorde shares a square with the target, it can also attempt to grapple or shove the target (resist/escape DC 14).

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) Piercing damage. If the target is not undead, it must succeed on a DC 14 Constitution saving throw or contract the disease Death's Shadow and be poisoned for as long as they are diseased. A diseased creature may repeat this saving throw upon completing a long rest, ending the condition on a success. On a failure, the creature takes (3d6) Necrotic damage and does not gain the benefits of a long rest. While poisoned in this way, if the target dies, it rises as a zombie in one minute.

Endless limbs. The Zombie Dreadhorde can make an attack of opportunity on every turn of combat.

The Zombie Dreadhorde can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Zombie Dreadhorde regains spent legendary actions at the start of its turn.

Detect. The Zombie Dreadhorde makes a Wisdom (Perception) check with advantage.

Dread Howl (Costs 2 Actions). The Zombie Dreadhorde releases a cacophony of terrible screams and howls invoking the mortality of one creature that it can see within 60ft of it. The target must make a DC 14 Wisdom saving throw. On a failed save, it takes (3d6) psychic damage and must immediately use its Reaction, if available, to move as far away from the Zombie Dreadhorde as it can, using the safest route. On a successful save, the target takes half as much damage only.

Overrun (Costs 3 Actions). The Zombie Dreadhorde moves up to half its speed. Each creature whose space either started within the Zombie Dreadhorde's space or that the Zombie Dreadhorde moves though must succeed on a DC 14 Dexterity or Strength saving throw or take 10 (2d6 + 3) bludgeoning damage and be knocked prone. Creatures that succeed this save take only half damage and are not knocked prone.