In one of my CKII games (Charlemagne start, the save is lost, sadly), AI Khazaria managed to conquer the entire Byzantine Empire in a single war, resulting in a nomad blob which began in Southern Italy and ended in Mongolia.
The Khazar ruler settled down, becoming a Muslim Byzantine Emperor and separating his realm from Khazaria. Several years later, he was overthrown by an Orthodox faction favouring the previous ruler. Afterwards, Khazaria, which lost about half of its territory to succession troubles, attacked and conquered the Byzantine Empire ``again``.
In the same savegame, Denmark conquered Lithuania and successfully converted it to the Germanic faith, and Poland decided to expand into pagan Sweden and Finland. Sweden regained its independence later on, but its rulers remained Polish.