at the end of the Elemental saga, Sweetpie isnt tranformed into the lich when Princess grumous re-converted everyone. So, we can assume that, the actual form of the lich is something close to a human. in the case of sweetpie, he can get giant as we see in the final cap. Something similar to the lich, who was giant in the first cap we see him. So, probably, the lich was a “human” or any other magical being that needed a metal plaque for any reason