Aberration is an alternate history scenario, in which the history starts to differ in various points. For example, for Finland it starts to differ in the 12th century rather radically (Swedish crusades fail and Finland becomes orthodox). For Byzantium the changes also begin in the 12th century and futher continue as the Fourth Crusade never happens. We will get alternate histories for all countries and show them in the country-description when you choose your country while starting Aberration. Some countries have totally fantasy monarchs and leaders (Finland), some have monarchs and leaders based on history, but going to fantasy quite soon (Byzantium, Brittany) and other countries have almost same monarchs as vanilla GC, but perhaps have different leaders. And at least all major countries have much new events, to make them majors. This scenario modifies only Europe though, so RotW is quite much the same as in vanilla GC. If you have more questions, feel free to ask.