fast zombie horde

My PCs have conquered the bad guys in “A Darkness Gathering” and now head off to the North in “Masters of Eternal Night“.

They are excavating a crater and revealing the wreckage of a crashed mind flayer planetesimal craft dating back to the days of the illthid empire. One of the rooms contains the planetesimals elder brain which is now an undead elder brain who has turned three of its lieutenant into mind flayer vampires.

After the PCs negotiated to “borrow” a vampire illithid (the mind flayer formerly known as Aphinox) they also gained a psychic whistle and the mind flayer promised more support once the Thralltakers attack.  This is the support Sempiternal provided.  I described a howling mob of mixed skeletons and zombies of mind flayers and giths that flowed around the PCs as they attacked the dumbfounded Thraltakers.

FAST ZOMBIE HORDE
A Gargantuan swarm of Medium undead (zombie), chaotic evil
Armor Class 12
Hit Points 192 (35d8 + 35)
Speed 40 ft.

Damage Immunities cold, necrotic, poison, psychic; bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, stunned
Senses Darkvision 60 ft., passive Perception 8
Languages Understands the languages of its creator but can’t speak
Challenge 5 (1,800 XP)

Pounce. If the fast zombie moves at least 20 feet straight toward a creature and then hits it with a slam attack on the same turn, that target must succeed on a DC 12 Strength saving throw or be knocked prone. If the target is prone, the fast zombie can make one bite attack against it as a bonus action – not active when a horde, included as part of Horde Mode under Actions.
Sunlight Sensitivity. While in sunlight, the fast zombie has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
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 beast. The swarm can’t regain hit points or gain temporary hit points. A swarm takes half damage from attacks that use an attack roll and double damage from effects that require it to make a saving roll. Creatures that are not swarms are impaired while they remain in the swarm’s space or within 5 feet of it and have disadvantage on attack rolls and Constitution saving throws to maintain concentration.
Undead Fortitude. If damage reduces the fast zombie to 0 hit points, that damage must be dealt by a headshot/decapitation, fire damage, or from a critical hit. Otherwise, the zombie drops to 1 hit point instead.

ACTIONS
Horde Mode. The swarm automatically deals 22 (6d6 + 1) piercing damage to any creature in the swarms’ space at the start of its turn, or 11 (3d6 + 1) piercing damage if the swarm has half of its hit points or fewer. Additionally, each creature must succeed on a DC 11 Strength saving throw or be knocked prone as the zombies swarm them.