I recently started a play-through with a hybrid between a Necromancer and an Assassin and I am trying to build up a party that can build an undead army. Eola is high on the priority list of companions as is Thralling necromancers (though I recently started this character so no access to Dead Thrall yet), but I was wondering who else would be good to create my undead horde (using UFO)? Any suggestions whether it be building my army or could be included for thematic reasons?

Now i know this doesn't exactly answer your question but since you're going for the whole undead horde thing, this mod may be of interest to you: http://skyrim.nexusmods.com/mods/9583