Complex sentence: Holmes reassures Carruthers the "marriage" performed by Williamson was void; not only was it performed against Violet's will, but Williamson had been defrocked and therefore had no authority to legalise a marriage.
Split into simpler sentences: Holmes reassures Carruthers the "marriage" performed by Williamson was void. Not only was it performed against Violet's will, but Williamson had been defrocked and therefore had no authority to legalise a marriage.