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 extra 5 (1d10) damage. If the target is a creature, it must succeed on a DC 13 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.

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

Mulitattack. The Zombie Swarm makes three attacks.

Slam. Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm’s space. Hit: 11 (1d8 + 3) bludgeoning damage