Czar  (OED, History, and Emily Dickinson [fl. ca. 1860-1886])

 a. Hist. The title of the autocrat or emperor of Russia; historically, borne also by Serbian rulers of the 14th c., as the Tsar Stephen Dushan.
E. D. CLARKE Trav. Russia, etc. (1839) 29/1 The connection which subsisted between the tsars of Muscovy and the emperors of Constantinople. 1890 MORFILL Russia 56 Ivan assuming the cognizance of the double-headed eagle, and partially taking the title of Tsar, the complete assumption of it being the achievement of Ivan IV. 1893 Times (Weekly ed.) 4 Aug. 606/4 Accident to the Tsar's yacht, the Tsarevna.

b. transf. A person having great authority or absolute power; a tyrant, ‘boss’. orig. U.S.
1866 in Sperber & Trittschuh Amer. Political Terms (1962) 111/1 There wuz an immense crowd, but the Czar uv all the Amerikas didn't get orf his speech here. 1893 McClure's Mag. I. 375 He was being held up as ‘The Czar’{em}a man whose iron heels were crushing out American popular government.

Czar Nicolas I (ruled 1826-1855), assassinated 1855 

Czar Alexander II (ruled 1855-1881), assassination attempts in 1866, 1879 (twice), and 1880.  Finally, in 1881, a group of four bomb-throwing assassins finally killed him

(The New York Times headline: "EXPRESSIONS OF SYMPATHY.; MESSAGES OF CONDOLENCE BEING SENT FROM ALL PARTS OF THE WORLD-- ENGLISH AND GERMAN PRINCES TO ATTEND THE FUNERAL.  March 15, 1881, Wednesday, Page 2.  BERLIN, March 14.--The sensation caused here by the assassination of the Czar is indescribable. The Emperor William wept bitterly at the terrible event, and is filled with the deepest sorrow. The imperial Princes remained with him until 2 o'clock this morning endeavoring to console him."