Relative clauses correspond to attributive adjectives (or participles), since like adjectives they serve to define substantives. Like adjectives, too, they often have the value of substantives and stand in any case.
δν γὰρ θεοὶ φιλοῦσιν ( = ὁ θεοφιλής), ἀποθνῄσκει νέος
ὧν ἔλαβεν ἅπα_σι μετέδωκεν
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].