So im running a chaotic campaign where the players are free to be the good guy or the bad guys of the story. Their main goal is to steal back this relic that belongs to a death goddess from a lich thats planning to use it to ascend to godhood and im planning to have the lich have a big **** off army of undead be raised up to defend him while hes doing the ritual. In order to ramp up the scale, lets just say that hes got an entire country and a half's worth of zombos to defend him and in order to counter that, the players will have to build up their own armies. Im trying to make sure that the players dont just rely on the human armies and i want them to branch out into other races. I need advice on what kind of other races or factions they could recruit. So far i have a hyper intelligent dire troll that leads an army of monsters to represent monstrosities and an ancient red dragon for draconic and her draconic brood but thats pretty much it.  

This is one thing I have never understood about the threat of zombies.  The shouldn't be that dangerous.  They have human teeth: human teeth suck as weapons.  As a comedian I once heard, remarked there are sandwiches that are too toasted for human teeth to tear into.  A fighter in chainmail with an axe should be just fine *forever* wading into population center full of zombies.  When he gets tired he can just lay down and sleep, those zombies will be unable to hurt him because of their human teeth.

You may now go back to your regularly scheduled thread.

Some sort of aberration or grand wizard that rips and tears holes into other realities to bring in various fiends, elementals, and aberrations.

And yeah, as hawksmoor said, zombos alone probably won't be much to overtake such varied monsters. you'll probably have to branch out into bigger, stronger, and smarter undead (ghouls, revenants, specters)

Im just afraid of adding something stronger than an ogre zombie because the lich could probably kill a solar and a pit fiend working together. Hes based off of a character from Warhammer so hes hella strong. Im talking about... owning Hazirawn , Book Of Vile Darkness and a Staff of the forgotten one. Plus his size is huge with almost double hp of a regular lich

Some kind of air support, like a kenku leading a group of giant eagles or hippogriffs would be a cool visual, and allow for scouting. 

A dryad and a bunch of forest creatures.

 
A mismatched group of paladins, one from each of the already existing gods, because they don’t want to see this power rise. 

An amphibious assault team made up of saughin (pretty sure I spelled that wrong, I hope you know what I meant).

A different lich with his own group of zombies, because he just doesn’t like the guy. 

If you have an underdark, you could have some drow, duergar, and sniferviblen. Either together or apart, to help with night time assaults ( since the zombies won’t need to sleep, but everyone else will)

Sahuagin. And yeah, you really need to add a Death Knight or something. Plus ghouls, vampires, wights, skeletons. And the incorporeals: specters, banshees, wraiths, ghosts, and will-o-wisps.