By the late 1670s, Parliamentarians were pushing to legitimize Monmouth’s claim to the throne by arguing that Lucy and Charles had been legally married. This effort became known as the affair of the "black box." It was alleged that Lucy, before her death, had placed her marriage records in a black box and given this to Anglican Bishop John Cosin, who was by this stage deceased and unable to testify. Witnesses swore they had seen the black box, although neither box nor papers were ever found. The affair created such a sensation in 1679-80 that Charles II was required to swear three separate oaths to the Privy Council that he had never been married to anyone but his queen.
King Charles II died in February 1685 and James II was crowned. In June, Monmouth returned to England to gather an army and declare himself "now head and captain-general of the protestant forces of this kingdom" with a "legitimate and legal" right to the crown, which he promised he would exercise only with the agreement of a free parliament. Monmouth was popular in the countryside. He had distinguished himself as a fine soldier in an impressive military career; and as a Protestant, he had the support of the anti-Papists. However, the widespread uprising expected by Parliamentarians did not occur. Monmouth’s forces engaged in several skirmishes, but were defeated within weeks at Sedgemoor near Bristol, where Monmouth was captured. He was executed in July 1685 in London.