What is the ruling of Islamic law and what is the legitimate advice for a person who did not tell his fiancée, to whom he is already married by contract, that he had previously suffered burns, fearing of losing her?

If the burn causes a congenital disfigurement that prevents full enjoyment, the afflicted person must inform his fiancée; otherwise, he would be deceiving her. In such a case, she retains the option to annul or uphold the marriage, as a woman can be returned for any defect that prevents full enjoyment.