Undead Fortitude. If damage would reduce the swarm below half 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 swarm drops to half of its total hit points plus 1 instead.

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 normally regain hit points or gain temporary hit points. Making an opportunity attack does not expend the swarm's reaction, but it may only make an attack of opportunity against a specific creature once per round.

Thousand Hands. The swarm counts as difficult terrain for creatures moving through it, additionally, creatures starting their turn inside the swarm must make a DC14 strength saving throw or be grappled.

The swarm can make a melee weapon attack against each creature within it and use absorb once on its turn.

Mob. Melee Weapon Attack: +6 to hit, reach 0 ft., one target. Hit: 15 (4d6 + 4) bludgeoning damage, or 9 (2d6 + 3) if the swarm has half its hit points or fewer.  The target must succeed on a DC14 strength saving throw or be knocked prone.

Absorb. The swarm absorbs one or more zombies in its space or adjacent to it, recovering hit points equal to the combined hit points of all absorbed zombies. The absorbed zombies are then removed from the encounter.