Grand Duchess Elena Vladimirovna of Russia (17 January 1882 – 13 March 1957), sometimes known as Helen, Helena, Helene, Ellen, Yelena, Hélène, or Eleni, was a Russian grand duchess as the only daughter and youngest child of Grand Duke Vladimir Alexandrovich of Russia and Duchess Marie of Mecklenburg-Schwerin.
 Her husband was Prince Nicholas of Greece and Denmark and they were both first cousins of Tsar Nicholas II of Russia.
The Greek royal family (Greek: Ελληνική Βασιλική Οικογένεια) is a branch of the House of Glücksburg that reigned in Greece from 1863 to 1924 and again from 1935 to 1973.
 Its first monarch was George I, the second son of King Christian IX of Denmark.
 He and his successors styled themselves "Kings of the Hellenes".
 Most members of the dynasty (aside from the last king to reign, Constantine II and his queen consort, Anne-Marie of Denmark) hold the title "Prince or Princess of Greece and Denmark" with the style "Royal Highness", except Marina, Consort of Prince Michael, and their daughters Princess Alexandra and Princess Olga.
In the mid-13th century the Hohenstaufen Holy Roman Emperor Frederick II made the County of Aosta a duchy; its arms were carried in the Savoyard coat of arms until the unification of Italy in 1870.
 The region remained part of Savoy lands, with the exception of a French occupation, 1539—1563.
 The title Duke of Aosta was given to various princes of the dynasty of Sardinia, second sons of the reigning monarch.
 It can be compared to the English Duke of York, French Duke of Orléans, Swedish Duke of Södermanland and the Scottish Duke of Albany.
 It remained in the branch of Prince Amedeo of Savoia, the second son of king Victor Emanuel II of Italy, as he was the first ever cadet prince Duke of Aosta who left male heirs.
 The subsidiary titles of the Duke of Aosta are, from the heritage of Maria Vittoria dal Pozzo, the mother of Duke Emanuele Filiberto, Prince della Cisterna and of Belriguardo, Marquess of Voghera, and Count of Ponderano.
 Ponderano was created in 1559, Voghera in 1618; Cisterna and Belriguardo as princely in 1670.
 The title has been used since July 2006 by Amedeo's son Prince Aimone, Duke of Apulia (born 1967), who is married to Princess Olga of Greece, younger daughter of Prince Michael of Greece and Denmark.
Prince Nicholas of Greece and Denmark (, 22 January 1872 – 8 February 1938), of the Glücksburg branch of the House of Oldenburg, was the fourth child and third son of King George I of Greece, and of Queen Olga.
 He was known as "Greek Nicky" in the family to distinguish him from his paternal first cousin Emperor Nicholas II of Russia.
 Prince Nicholas was a talented painter, often signing his works as "Nicolas Leprince."
George I (Greek: Γεώργιος Α΄ , "Geórgios I"; born Prince William of Schleswig-Holstein-Sonderburg-Glücksburg; 24 December 1845 – 18 March 1913) was King of Greece from 1863 until his assassination in 1913.
Prince Nicholas of Romania (Romanian: "Principele Nicolae a României" ; 5 August 1903 – 9 June 1978), later known as Prince Nicholas of Hohenzollern-Sigmaringen, was the fourth child and second son of King Ferdinand I of Romania and his wife Queen Marie.
Sokol Baci (1837–1920) was the chief of the Gruda, a northern Albanian tribe in the vicinity of Podgorica (now Montenegro).
 Originally, he had served the Ottoman sultan in his personal guard, but switched sides after he was maltreated, and fought the Ottoman forces in the Sanjak of Scutari.
 After his clan was defeated and subjugated, he was exiled and sought refuge in Montenegro, even though he had earlier fought against them in the 1870s, and lived in Podgorica beginning in approximately 1884.
 Prince Nicholas I of Montenegro recognized his status and employed him.
 He was one of the leaders of the Albanian Revolt of 1911, alongside chiefs such as Ded Gjo Luli, Mehmet Shpëndi, Mirash Luca and Luigj Gurakuqi.
 In 1912, the entire tribes of Gruda and Hoti, along with major portions of the Kastrati, Shkreli, and Klimenti tribes, backed Montenegro during the Balkan Wars.
 In 1913, he was appointed commander of Scutari by King Nicholas I of Montenegro, where he lived until his death in 1920.
Grand Duchess Alexandra Georgievna of Russia (Russian: Алекса́ндра Гео́ргиевна ); "née" Princess Alexandra of Greece and Denmark (Greek: Πριγκίπισσα Αλεξάνδρα της Ελλάδας και της Δανίας ); 30 August 1870 – 24 September 1891) was the third child and firstborn daughter of King George I and Queen Olga of Greece, who herself was a daughter of a Russian grand duke, and was also a grandchild of Denmark's King Christian IX and Queen Louise.
 She was a sister to Constantine I of Greece, and thus aunt of three kings and two queens, Constantine's three sons, who all became kings of Greece, and two of his daughters, who were queens, in name, of Romania and Croatia, respectively.
 She was also first cousin of Tsar Nicholas II of Russia, King George V of the United Kingdom, and both King Haakon VII and Queen Maud of Norway, as well as a paternal aunt of Prince Philip, Duke of Edinburgh.
Princess Marina of Greece and Denmark, {'1': ", '2': ", '3': ", '4': "} (Greek: Πριγκίπισσα Μαρίνα της Ελλάδας και Δανίας ; 13 December [O.S. 30 November] 1906 27 August 1968) was the daughter of Prince Nicholas of Greece and Denmark and Grand Duchess Elena Vladimirovna of Russia.
Princess Elisabeth of Greece and Denmark (Danish: "Elisabeth af Grækenland og Danmark" , Greek: Πριγκίπισσα Ελισάβετ της Ελλάδας και Δανίας or Ελισάβετ της Ελλάδας ; 24 May 1904 – 11 January 1955) was the middle daughter of Prince Nicholas of Greece and Grand Duchess Elena Vladimirovna of Russia.

Question: Prince Nicholas of Greece and Denmark was the son of which king that was assassinated in 1913?  
Answer: George I