I just can't seem to turn people into undead, even though I have the building in the Necropolis castle! Maybe I haven't learned a certain spell? I don't want to replay the entire Necromancer campaign, I've already replayed it once to develop the "Mind Control" magic, but I still haven't learned how to lure Titans or anyone else to my side...
Guys, don't be mad, just tell me, what's the mistake?

Titans are immune to mind control magic, they can't be lured... unless you get the Amulet of the Gods, it's some kind of artifact, a cloak... I don't remember exactly. And you probably don't have enough developed magic, you just haven't gotten the spell yet; it's given as a bonus for some mission... or you can learn it in the city, but don't forget to build magic towers!