Usage patterns
〈人〉の家臣になるto become someone's vassal
He became a vassal of Oda Nobunaga.
家臣を召し抱えるto take someone into service as a retainer
The daimyo employed many retainers.
家臣が主君に忠誠を尽くすfor a retainer to devote loyalty to his lord
The retainers pledged their loyalty to their lord.