[[WMG: The spirit of the cauldron was what turned Luna into nightmare moon ]]
It seems that it had great power beyond what it utilized, perhaps it corrupted Luna...
** Keep in mind that these are two non-related universes, with no common points. There is also the temporal issue, in that anything after the film would be using a useless cauldron whose dark magic was nullified while anything before the film began takes place in the film's presented back-story. Additionally, Luna's conversion into Nightmare Moon was perfectly explained in that she wanted to be respected and to achieve this gave into using her powers through her own darkness thinking that as the new ruler she would have the respect heaped upon Celestia. King Sombra might be less-absurd but then you run into the problems of combining the two non-overlapping universes.