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 historian and moralist Lord Actonn (1834–1902). He 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.
More in-depth description here.