When you've thrown everything but the kitchen sink in the grand pool of monsters roaming across the land, when you have scary monsters, and then worse monsters, who are ruled over by creatures of the abyss, who can be taken down by weird, soulless things, which can be easily crushed by even bigger monsters - well, all you need then is zombies, I suppose.
I mean, they have some bodies of former really strong warriors lying about, surely then *can* be put to good use.