Absolute monarchies are those in which all power is given to or, as is more often the case, taken by, the monarch. Examples of absolute power corrupting are Roman emperors (who declared themselves gods) and Napoleon Bonaparte (who declared himself an emperor).
"Absolute power corrupts absolutely" arose as part of a quotation by the expansively named and impressively hirsute John Emerich Edward Dalberg Acton, first Baron Acton (1834–1902). The historian and moralist, who was otherwise known simply as Lord Acton, expressed this opinion in a letter to Bishop Mandell Creighton in 1887:
"Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men."
The text is a favourite of collectors of quotations and is always included in anthologies. If you are looking for the exact "power tends to corrupt, and absolute power corrupts absolutely" wording, then Acton is your man. He didn't invent the idea though; quotations very like it had been uttered by several authors well before 1887. Primary amongst them was another English politician with no shortage of names - William Pitt the Elder, Earl of Chatham and British Prime Minister from 1766 to 1778, who said something similar in a speech to the UK House of Lords in 1770:
"Unlimited power is apt to corrupt the minds of those who possess it"
Acton is likely to have taken his lead from the writings of the French republican poet and politician, again a generously titled individual - Alphonse Marie Louis de Prat de Lamartine. An English translation of Lamartine's essay France and England: a Vision of the Future was published in London in 1848 and included this text:
It is not only the slave or serf who is ameliorated in becoming free... the master himself did not gain less in every point of view,... for absolute power corrupts the best natures.
Whether it is Lamartine or his anonymous English translator that can claim to have coined 'absolute power corrupts' we can't be sure, but we can be sure that it wasn't Lord Acton.