View Solution

Level: Hard
Start by obtaining enough gold to create the Duchy of Calabria and then the Kingdom of Sicily, then focus on Kingdom of Epirus.

The achievement does not require you to be independent, so it is possible to swear fealty to the Byzantine Empire, obtain the Religious Rights feudal contract via hook, and conquer and convert the required provinces as a vassal.

Alternatively, educate your heir as Greek and Orthodox, claim the emperor's throne with him, and then convert back to Catholic with your vassals, taking care not to lose the Kingdom of Sicily, Naples or Trinacria.

Or you can get enough prestige to start an invade kingdom war for one of the three kingdoms and then start coverting all counties and new vassals there. Before you invade the empire you can kill the emperor to remove his allies and try to get some strong allies for yourself.

Credit to Bayek von Kuwaq on Steam for his guide.