I've been playing StarCraft for the past few days, and it's got me thinking about how to run a good "zombie horde" style campaign, in which an army of slow-moving, animalistic foes attacks the PC's community and begin to (in some way) assimilate the general populace into their ranks. Zombies are the most common monster to fill this role, but really any monster will do with the right evil master plan (Far Realm abominations have served this role quite well for me).

The archetypal plot for the zombie horde, as I've used it and seen it used in published adventures, is that an outlying, rural town reports being attacked and some of their number are missing, after which the PCs are sent out to take care of this seemingly minor problem. Once they actually arrive in this town, however, the PCs discover that the town is either under siege or overrun with the walking dead (or whatever your monster of choice is) and the PCs must rush to its defense.

StarCraft does something interesting in this regard. In StarCraft, the zerg fill the role of zombies, and you know there's a horde of them from the beginning. Rather than accidentally stumbling upon an unruly mass of zerg, you pretty much know they're usually lurking around, and you even use them to destroy your enemies at times. It's only after you've let them creep across your territory that you begin to consider them the serious threat that they are.

I find this archetype very intriguing. Instead of happening upon a horde of zombies, what if the PCs know there are undead wandering the wilderness near their home city all along? Not a "horde," per se, but it's not uncommon to run into these zombies on a trek through the woods. The PCs know they should deal with the zombie threat at some point, but there are always more pressing matters to deal with. That is, until they go to another city and find that the same horde is harassing guard details there too... and there are reports of the same zombies attacking cities all across the continent. Using this archetype, the PCs don't stumble into a den of undeath and heroically defend a ravaged community; they irresponsibly neglect a threat until it grows out of control.

And finally, because the Monster Manual zombies aren't nearly as handsy as I'd like:

Horde Zombie        CR 2
NE Medium undead
Init +0; Senses Listen +1, Spot +1; darkvision 60 ft.
Languages —
DEFENSE
hp 26 (4 HD); DR 5/slashing
AC 15 (+5 natural), touch 10, flat-footed 15
Immune undead immunities
Resist cold 5
Fort +1, Ref +1, Will +5
Weakness critical vulnerability, vulnerability to fire
OFFENSE
Speed 20 ft. (4 squares; can't run or charge)
Melee slam +6 (1d8+4) or
    zombie grab +6 touch (see below)
Space 5 ft.; Reach 5 ft.
Base Atk +2; Grp +6; see horde tactics
Special Actions zombie bite (1d6+4 plus walking death), zombie grab
STATISTICS
Abilities Str 19, Dex 10, Con —, Int —, Wis 12, Cha 1
Feats —
Skills —
ABILITIES
Critical Vulnerability (Ex) When rolls a natural 20 on its attack roll against the horde zombie, that opponent may roll to confirm as though it were a critical threat. If the opponent confirms this roll, the horde zombie is automatically destroyed.
Horde Tactics (Ex) A horde zombie gains a +1 bonus on grapple checks for each other horde zombie adjacent to the target of the grapple.
Walking Death (Su) Supernatural disease—zombie bite, Fortitude DC 16, incubation period 1 minute; damage 1d4 Con and 1d4 Int. The save DC is Strength-based.
    Unlike a normal disease, the victim of walking death must make a saving throw each hour or suffer the disease's effects again. In addition, walking death requires three successful saving throws in a row (rather than two) to fight off naturally.
    While suffering the effects of walking death, the victim only gains half the benefit of any conjuration (healing) effect cast upon him. However, should the victim suffer damage from a negative energy attack (such as the inflict spells), he heals a number of hit points equal to half the damage that would have been dealt.
    Additionally, an afflicted creature who dies while suffering the effects of walking death turns into a horde zombie 1d4 rounds later.
Zombie Bite (Ex) A horde zombie deals 1d6+4 bludgeoning, piercing, and slashing damage with a successful grapple check. In addition, the target suffers the effects of the horde zombie's walking death ability.
Zombie Grab (Ex) The horde zombie can perform a melee touch attack against a foe within reach. If the attack succeeds, the horde zombie can attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can use zombie bite.