Zombie Mass (3.5e Variant Rule)
From D&D Wiki
You've all watch zombie movies, that moment when one of the characters is pulled down by the mass of zombies and eaten is iconic. How does that work in game? Well here are some ideas:
1. More than one zombie can occupy a 5-foot square: Up to 15 Small or 10 Medium size zombies can occupy one square.
2. Zombies get a negative to hit when more than one is occupying the same square. Broken down like this; 1-2 Zombies: no penalty, 3-4 Zombies: -1 to hit, 5-6 zombies: -2 to hit, 7-8 zombies: -3 to hit, 9+ zombies: -4 to hit.
3. The more zombies in a square, the easier time they have grappling! For each zombie past the 3rd the zombies get a +2 to their grapple checks.
4. Getting critically hit by this mess is dangerous! If one zombie in the square critically hits, there is an immediate grapple check initiated.
5. Getting grappled by zombies sucks! If a character is grappled by a mass of zombies in one five foot square they automatically take a random number of bite attacks equal to a die roll of the number of zombies in the square - i.e. if the square holds eight zombies the character grappled takes 1d8 bite attacks each round. This can get deadly quickly!
6. Critically hitting zombies in this predicament can be a good thing! Despite being immune to critical hits, in this situation the zombies have a real problem. If you manage to kill the zombie you hit with the natural 20 you automatically can cleave, per the feat, another zombie in that same square. If you already have the cleave feat you can continue attacking zombies in that square as if you had Great Cleave.
Of course these rules may make what on the surface is an easy encounter potentially quite deadly, so the DM should up the xp reward accordingly.