You'd think someone as manipulative as Klaus wouldn't be so easy to manipulate, either. He's spent all this time reclaiming New Orleans for himself, and now the witches have him off on some pointless revenge quest so they can take over again while he isn't paying attention. Honestly I wish the crazy sacrificing guy had been the Big Bad, instead of Celeste. Maybe we'll find out her biggest weakness is frozen pizza. As for how they became so powerful, they're using the Harvest magic combined with the energy Captain Cobra sucked out of all the vampires he killed.
We haven't been given any explanation for how she went from Elijah's beloved to a crazy body snatcher, and if body snatching was so easy, you'd think every witch would be doing it. (Of course, everyone seems to have completely forgotten at this point that body snatching was how Klaus revealed himself in the first place, when he possessed Alaric. These writers seem to use it as their go-to plot device, and TVD is still using it as we speak.)