In the same vein, going from being a lord of a paltry 2 counties to owning a Kingdom consisting of 14 instead. Modest in the grand scheme, but still impressive.
Installing your dynasty onto the throne(s) of great kingdoms and empires, bringing prestige to your family name.
All of this goes double if you manage it within a single lifetime. Imagine being born as some count in a relatively impoverished backwater, and ending your days as one of the world's foremost rulers.
Victory through intrigue! Instantly ending an unwinnable claim war by assassinating the claimant. Isolating your enemies by assassinating the spouse who secures a powerful alliance. Tywin Lanister or the Salarians would be proud.
The Expansions (Sword of Islam, Sunset Invasion,The Republic, The Old Gods, Sons of Abraham, Rajas of India)