Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for an adult humanoid. The swarm can’t regain hit points or gain temporary hit points.

Trampling Charge. If the Zombie Swarm moves at least 15 ft. straight toward a target and then hits it with a Slam attack on the same turn, the target takes an extra 5 (1d10) bludgeioning damage. If the target is a creature it must succeed on a DC13 Strength saving throw or be knocked prone. If the target is prone the Zombie Swarm can make another Slam attack against it as a bonus action.

Multiattack. The Zombie Swarm makes three attacks

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d8 + 3) bludgeoning damage.