Arthur Gerald Wellesley, Earl of Mornington, is the great-great-grandson of what  last German Emperor ("Kaiser") and King of Prussia?
Wilhelm II, German Emperor Wilhelm II or William II (German: "Friedrich Wilhelm Viktor Albert von Preußen", English: Frederick William Victor Albert of Prussia; 27 January 18594 June 1941) was the last German Emperor ("Kaiser") and King of Prussia, ruling the German Empire and the Kingdom of Prussia from 15 June 1888 to 9 November 1918. He was the eldest grandchild of the British Queen Victoria and related to many monarchs and princes of Europe.
Duke of Marlborough (title) Duke of Marlborough ( ) is a title in the Peerage of England. It was created by Queen Anne in 1702 for John Churchill, 1st Earl of Marlborough (1650–1722), the noted military leader. In historical text, it is often to him that an unqualified use of the title refers. The name of the dukedom refers to Marlborough in Wiltshire. It is one of the few titles in the peerage which allows for "suo jure" female inheritance, and the only current dukedom to do so.