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The players in my current campaign are deep in the crypts of Death Frost Doom. They have woken the dead, and spurned the friendship of one creature that might have been able to help them. I’ve heard the players talk about wading into the undead horde and attempting to cut their way to the surface, so I decided I needed to know how I would handle that in game mechanical terms. The basic idea that I settled on is to treat the entire horde as a single creature with a huge number of HP and a variable number of attacks depending on the disposition of the undead horde front lines.

Assuming Moldvay stats for zombies, the horde will have 2 HD (9 HP on average) times the number of undead in total. The undead are shoulder to shoulder, approximately 2.5 per five feet of front (round up). So if a horde was surging up a corridor 10 feet wide, the front would be 5 zombies wide. Putting down an HP total of zombies equal to the front line of undead will push back the horde five feet and create terrain difficult for PCs (movement rate is halved), though the terrain is not considered difficult for the horde. The horde will advance 15 feet per round.

The horde will surge as one toward any source of life and flesh. Due to the close nature of the horde, it will take extra damage from area attack or grenade-like weapons, as they are more likely to catch more undead within their blast radius. The horde should be considered to automatically fail any saving throw associated with such an attack, and any damage is doubled. Examples of such attacks are flaming oil, explosives, and fireballs. The confined zombie horde should only be treated like a single monster while it is confined; if it breaks out into the open for whatever reason, encounters should again be run as with individual undead.

Horde attacks:

• The horde front line gets six attacks against any adventurer foolhardy enough to engage in melee. This number of attacks increases as the horde advances around the character. For example, 10 attacks if the throng advances five feet, 16 once the character is surrounded.
• Surge and trample: if at least two front line attacks hit, the enemy is knocked prone and pulled under the horde. On the next turn the monsters will advance over the character as if the area was unoccupied, and the horde will make nine attacks against the overwhelmed target. These attacks are in addition to the standard attacks made by the horde front lines against any other targets.
• Jumper: once per round, optionally, 1d4 zombies (adjust for situational logic) from the rear ranks will clamber over the shoulders of the front lines and fling themselves at any living creature nearby. Range is 20 feet, and if the attack hits it will do double damage.
• Any PCs overcome and reduced to 0 HP by the horde will be torn apart and eaten, and thus the PC corpse will not be recoverable.

Holding a line against the horde is difficult to impossible, as the undead will just sacrifice their front line in order to overcome the defense (for example, zombies will impale themselves on set spears, probably disarming and spear wielders in the process). Make sure to deduct HP from the horde whenever individual undead detach themselves in addition to when the horde itself is damaged.

The Death Frost Doom horde (at least the part underground) is 9885.5 HD, or 44,484 HP (assuming the average of 4.5 HP per die). I was nice and rounded down.

What do you think, is this horrific enough to represent a zombie multitude?

Image from Wikipedia

Any advance movement by the horde is considered to be shifting (that is, it does not provoke opportunity attacks). Forced movement that would move the horde away from characters is ineffective (the mass of undead behind preventing any reversal), though at the referee’s discretion such forced movement may decrease the number of horde attacks during the next turn as the “fleeing” undead will act as an obstacle to other zombies. Single zombies may be pulled away from the horde as normal and should then be treated like individual creatures again until they rejoin the horde (which should be considered to happen automatically if the horde advances).

Characters enveloped by the horde are considered prone and restrained (and grant combat advantage to the horde). Zombies may also target any defense (AC, fortitude, reflex, or will) when attacking characters that have been overcome, and will generally target the most vulnerable defense. Normal attacks are +5 versus AC, and the horde defense are AC 17, fortitude 20, reflex 10, and will 15.

I consider each hit die to be worth 10-15 HP in Fourth Edition, so total horde HP is 98,855.