Baroness Emmuska Orczy 
I Will Repay 

This is a sequel novel to the 'Scarlet Pimpernel.’ The novel revolves around Juliette Marny and Paul Déroulède. On hearing of the death of his only son, the Duc de Marny is distraught. The Duc summons his daughter, Juliette, to his side and forces her to swear an oath to ruin Déroulede in revenge for her brother’s death, telling her that her brother’s soul will remain in torment until the final judgment day should she break her promise. Ten years later, when she finally meets Paul Déroulède, she finds out the truth about her brother’s death. Will she honor her oath or protect the man she finally loves?