A relative pronoun introduces a relative clause and refers back to another nominal as its antecedent. Common relative pronouns are who, whom, which and what.
A relative pronoun refers back to another nominal as it’s antecedent and introduces a relative clause. Who, whom,what and which are common relative pronouns.
ἐπίστευσαν τῇ γραφῇ καὶ τῷ λόγῳ ὃν εἶπεν ὁ Ἰησοῦς. (Jhn 2:22)
they believed the Scripture and the word which Jesus said
A relative pronoun always agrees with its antecedent in gender and number, but not in case. The case is determined by its function in the relative clause (see ###advanced).
A relative pronoun sometimes takes on the same case as its antecedent, 1) when it is in close proximity to its antecendent and 2) when the relative pronoun would normally be accusative but has been changed to match a genitive or dative antecedent.
ὑμεῖς ἐστὲ οἱ υἱοὶ τῶν προφητῶν καὶ τῆς διαθήκης ἧς ὁ θεὸς διέθετο πρὸς τοὺς πατέρας ὑμῶν (Act 3:25)
you are the sons of the prophets and of the covenant which God gave to our fathers
In this verse, the relative pronoun ἧς (of which) is genitive, to match its antecedent, τῆς διαθήκης (of the covenant), though it should be accusative ἥν (which) since it is the direct object of the verb (διέθετο, [God] gave).