The Hero, or a member of the heroic band, finds a Mentor with new secret techniques to teach. The student eagerly signs on, only to learn later that there's a catch — the mentor is evil, has a hidden agenda of his or her own, and those new abilities are seriously nasty (though certainly not useless). The student may feel "soiled" by having learned these techniques and might swear off ever using [...] [tvtropes.org]