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.

Infection. If a living creature is hit by Slam then DC 10 STR,DEX or CON to not get infected. If the creature fails then it loses 1 Max HP per day until Max HP is 0 which will kill the creature. When the creature dies it rises as a new zombie with 22 HP instead of 1 HP. The creature can turn sooner if it dies outright before Max HP drain kills it. To fix infection two things can be done

1. Greater Restoration spell
2. Remove Infected limb before long rest

The Horde. Loud noises attract 1D4 zombies and really loud noises attract 1D10 zombies

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.