Added 4 minutes later
From memory, the author gives us the Cloak of the Undead King at the very end of the map, and possibly a Necropolis hero (I don't remember exactly if the hero is given or needs to be leveled up). On the legion garrisons, you can and should use the Cloak of the Undead King to gather a proportionate number of liches for the final boss's lich stacks. I.e., this is a very clear indication of how to build an army against the final boss. Without liches, IMO, he's unkillable.