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

Swarm: Swarms have the swarm creature trait, which means that the swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for any one individual without squeezing. may not gain hit points or temporary hit points. However, if two swarms are within the movement range of one another, you may transfer any number of individuals from one swarm to another on your turn (no action required by you), increasing their effectiveness.

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 18 (1d6 + 15) bludgeoning damage. Damage modifier gets +1 for each Zombie in the Swarm. (This group has 15)