Well, to create some more unique undead (Boneyard example) need rituals combined with create undead spell and certain CL. So far what I have seen suggested is that you need CL 20 and hundred individual skeletons along with create undead led ritual to create it (and ritual takes/happens at a graveyard). Details can be discussed how rituals are done but instead list every undead it's easier ask 'what you want?' as undead minions.