King of Kings, Ruler of Rulers is a decision that can be taken by 
Greece in order to restore the 
Byzantine Empire, which fell to the Ottomans in 1453.
Greece, as a major power, has to control Constantinople (Istanbul), most of the western half of Anatolia, along with mainland Greece, to take this decision. Additionally Greece has to be either an absolute monarchy, Prussian Constitionalism or HM's Government. Upon taking this decision all non-colonial states held by Greece (now Byzantium) that have Greek or Turkish culture will be instantly cored, along with prestige and -10 infamy. It will also change the capital to Istanbul and rename it Constantinoble.
The decision cannot be seen before Greece is a great power, and is therefore considered an easter egg along with 
Babylon.
With the collapse of the Ottoman Empire, the Ottoman claims to Imperial Roman titles, so viciously and deceitfully wrought from Greek hands in the dark year of 1453, are all fast becoming fading memories. Constantinople, the City of Man's desire, is again, after centuries of Turkish occupation, in Greek hands. As years and time passed again, again ours they were. Basileos, proclaim our empire restored!