孽 is a bound form (b.f.) — it never stands alone in modern Chinese; you always see it as part of literary, religious, or legal compounds. Its core meanings span from a literal 'monster' or 'demon' (often in Buddhist contexts) to a moral 'sin' or 'evil deed' (罪孽), and historically from 'illegitimate child' (孽种). The word carries heavy cultural weight in classical literature and Buddhist karma-related vocabulary.