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

Mob. The mob of zombies can occupy another creature's space and vice versa, and the mob can move through any opening large enough for a Medium zombie. The mob can't regain hit points or gain temporary hit points. The mob makes its attack rolls with Advantage  

Hungry. If a creature ends their turn while inside the mob, they must succeed on a DC 12 Strength saving throw or be knocked prone. While knocked prone in this manner, the creature is considered grappled by the mob. A creature grappled in this way can make a DC 12 Strength (Athletics) check at the start of their turn to break the grapple. 

Multiattack. The Zombie Mob makes two slam attacks with advantage.

Slam. Melee Weapon Attack: +4 to hit, reach 0 ft., one target. Hit: 4 (2d6 + 2) bludgeoning damage, or Hit: 4 (1d6 + 2) bludgeoning damage if the mob has half it’s hit points or fewer.