Zombie outbreaks in a game of In Nomine are different from most other settings because a horde of zombies poses no direct threat to an angel. Not only is the angel massively more powerful (9 Force vs. 4 Force), but zombieism is non-communicable.
This doesn’t mean that the zombie outbreak is an unusable scenario, but you will need to adjust the setting and add some additional objectives or threats.
In the modern zombie film, anyone who is bitten (or sometimes clawed) by a zombie is themself in danger of becoming a zombie.
I would treat this as a strong, very fast disease (see the Corporeal Players’ Guide for full disease rules). Anyone who dies before they’ve stopped damage from the disease will also turn into a zombie. The GM can determine how long this transformation will take.
Until a treatment is found, successful Medicine rolls can only add 1 to Strength rolls made to resist the disease. As always, angels are immune to mundane diseases, but that just means the angels need to be worried about more than their own survival.
Protect the Human(s)
The party is responsible for protecting a community of humans, or perhaps just one human in particular (someone of particularly strong faith, or whose scientific acumen means that they can halt the spread of disease borne zombieism). While the angels may be in no real danger, the same cannot be said of their charges.
Contain the Outbreak
Rather than worrying about any particular community of humans, the party is responsible for keeping the affliction from spreading to humanity as a whole. This may mean sacrificing some humans for the greater good of humanity (expect lots of dissonance rolls).
Track Down the Source
Maybe it’s a patient zero. More likely it is some Infernal or Ethereal who has learned how to change the nature of zombieism. Either way, the party has resolved to track down this origin so that they can make sure efforts to contain and cure the outbreak are not intentionally thwarted.
Push an Agenda
Perhaps the dead rising is just what humanity needs to regain it’s faith, build solidarity, start thinking creatively, &c. The party may not be responsible for the zombie outbreak, but they could still be ordered to find a way to make it serve heaven.
Find a Cure
The canonical version of In Nomine’s zombieism is neither an infection nor curable. However, it is not particularly prone to “outbreaks” either. If you’re running an outbreak scenario, zombieism may not follow the usual rules, and that means there may be a way to make its victims human again.
The presence of so many zombies is wrong – until the party resolves the issue, they will gain one point of dissonance every week. Variations include dissonance when witnessing a human become a zombie or dissonance whenever the angel is themself bitten or clawed by a zombie.
While each individual zombie is mindless, as a horde they begin to develop predatory cunning. For each additional zombie in a group, add 1 attribute to the horde. Start with Strength, then Agility, Intelligence, Precision, Perception, and finally Will. 18 corporeal hits destroys one zombie (reducing the total attributes by 1).
Whatever arcane force gave zombieism the ability to move from one mortal to another has also given it the ability to infect vessels. Should an angel’s vessel become terminally infected, the angel will be ejected same as if they had allowed their vessel to die, and everyone else will be stuck fending off an 8F zombie.
Someone is working against the party. Likely candidates include the source of the new zombieism, opportunistic demons, or a collective intelligence emerging from among the horde of zombies itself.
One Off vs. Campaign
For a one off adventure, the party only really needs to meet one of the objectives. Their success or failure will determine if zombies take over the earth, or if other humans and celestials are able to lick the zombie menace.
In a campaign, the zombies don’t get taken care of until the party takes care of them. That means the PCs will eventually need to contain the outbreak, find the source, or develop a cure (or perhaps find/protect the human who can).
Either way, I’d love to hear your thoughts on and experiences with this material.