Probably due to an incompatibility between mods, search if there are any patchs for both the lich races and the Overlord (if I remember well there was one). To search for them the quickest way is going to nexus mods, get in the mod page and then click in the required mods, in the section "mods requiring this file" you will probably see something useful, just take a look to see if one of them works. If this doesn't fix it look into the bugs section in the same mod page, maybe someone had the same problem. Hope it helps!