Swarm. The horde can occupy another creature's space and vice versa, and the horde can move through any opening large enough for a Medium zombie. The horde can't regain hit points or gain temporary hit points.

Undead Fortitude. If damage reduces the zombie horde 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 horde drops to 1 hit point instead.

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target in the horde's space. Hit: 14 (4d8) bludgeoning damage, or 7 (2d8) bludgeoning damage if the horde has half or fewer of its hit points or fewer.