Horde's Fortitude. If the swarm takes 5 or more damage in a single attack, 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 damage is reduced by a number equal to the damage taken divided by 5 (rounded down).

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

Multiattack. The swarm can make a slam and bite attack or two slam attacks on its turn.

Slam. Melee Weapon Attack: +6 to hit, reach 0 ft., one target. Hit:  12 (3d6 + 2) bludgeoning damage or 6 (1d6 + 2) bludgeoning damage if the swarm has half of its hit points or fewer.

Bite. Melee Weapon Attack: +6 to hit, reach 0 ft., one target. Hit:  7 (2d4 + 2) piercing damage or 4 (1d4 + 2) piercing damage if the swarm has half of its hit points or fewer. The target must make a DC 10 Constitution saving throw, taking 5 (2d4) poison damage on a failed save, or half as much damage on a successful one.