Undead Fortitude. If damage reduces the zombie horde by 24 hit points or more, it must make a Constitution saving throw with a DC of the damage taken - 12, unless the damage is radiant or from a critical hit. On a success, the zombie horde regains 2d6 hit points.

Braindead. A horde needs to be controlled by multiple wights, determined by the GM. If the wights lose control of the horde or are killed, the horde rampages giving  -2 decrease to hit but +16 points to damage rolls. If Wights are in control of the horde, the horde regains 2 hit points on the start of all of its turns. 

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a medium size creature. 

The Horde makes 3 slam attacks

Slam(Horde has more than half health). Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 36 (8d6 + 8) bludgeoning damage.

Slam(Horde has less than half health). Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 18 (4d6 + 4) bludgeoning damage.