Mayerling is the name of a hunting lodge in Austria, most well known as the site of the Archduke Rudolph of Habsburg's death, along with that of his mistress, Baroness Marie Vetsera, on January 30, 1889.
The two deaths were originally thought to be a lovers' death pact, either a double suicide or a murder and suicide, with the Archduke killing Vetsera with his pistol, then himself. Because of the public nature of the victims, there was most likely a cover-up by the police.
A 1996 examination of Vetsera's remains showed no bullet damage, only skull trauma as from a garden tool or blunt object. This has led some historians to believe that there was no lovers' pact, only an extraordinarily subtle assasination of the heir to the throne of Austria-Hungary.
There are two very good films detailing the popular version of what happened at the Archduke's lodge that winter. Both named "Mayerling", the first was a 1936 feature directed by Anton Litvak starring Charles Boyer and Danielle Darrieuz. It is in French with English subtitles. The second is a 1968 offering from the UK, starring Omar Sharif and Catherine Deneuve, directed by Terence Young. It is a faithful remake of the 1936 film.