Complex sentence: Garrick, although successful under Giffard, was rejected by the managers of the Covent Garden Theatre upon his return and so took up an engagement later that year at Drury Lane in what was to became perhaps his best known play "Richard III".
Split into simpler sentences: Garrick, although successful under Giffard, was rejected by the managers of the Covent Garden Theatre upon his return and so took up an engagement later that year at Drury Lane. Later that year, Garrick appeared again at the theatre in what was to become perhaps his best known play "Richard III".