Rudolf, [1858-1889] -- Archduke, Crown Prince of Austria, Hungary and Bohemia

Wiki: Rudolf, Crown Prince of Austria

His death, apparently through suicide, along with that of his mistress, Baroness Mary Vetsera, at his Mayerling hunting lodge in 1889 made international headlines, fueled international conspiracy rumours and ultimately may have sealed the long-term fate of the Habsburg monarchy. Despite numerous suicide letters, later forensic analysis concluded beating, not gunshots, for her death, so the exact cause and circumstances of Rudolf's death remain a mystery to this day.  
 

• Mistress: ¤VETSERA, Mary [1871-1889] Mysterious violent death, Baroness