Oath of Desecration is a low level witchcraft spell that's very powerful, especially when used on a two-handed man-at-arms user who has whirlwind.
Otherwise I dunno it seems kind of underwhelming compared to other skills. I guess it's because it is supposed to be more unconventional?
You should be able to beat the game completely ignoring certain skills in your party because it's "classless, play how you want"
