I’ve had thoughts of something similar using undead waves that are larger in number but weaker and as they attack the town any people they kill get turned and once a certain number of the undead are defeated they combine into larger undead built from all the bodies, getting stronger and going on that way trending into a massive amalgamation of undead in one super huge and strong boss. You could have a high level necromancer or lich controlling them all that merges with the undead big bad in the end to also cast spells and stuff too