Skeletor is already a powerful magic user. What if Evil Lyn was a vampire, with magic abilities secondary?
Here’s how you reign that power in a bit: 1) It doesn’t work on Skeletor…for whatever reason (he’s undead, no emotions, too powerful, etc.)
2) Skeletor’s magic can break the charms she has on others. He is the most powerful evil magic in this scenario. Therefore his minions are safe (Beastman, MerMan, etc.)
3) None of her thralls would be as powerful as any of the individual Masters. Anyone as powerful as the Masters would be able to shake it off relatively quickly, therefore anyone she can control longer term must be weaker.