A descendant of Europe’s richest family has reportedly begun a process to adopt his middle-aged former gardener, planning to leave him at least half of his roughly €12 billion ($13 billion) fortune.

Former Hungarian fencer László Csongrádi has been left so distraught by the theft of his Olympic gold medal that he has offered to leave it to the thief in his will if it is returned. Csongrádi, 65, ...