This is a pretty easy achievement to get but it does depend on a bit of luck. Also make sure ironman is enabled before you start, as you need it for achievements.
So the character you want to use is King Alfonso VI of Leon in the 1066 start (He is also a featured character in the Iberia bookmark). His kingdom is located in the northern part of Spain.
There are a few reasons for this, One is you need to play as a Jimena sibling and become emperor of Spain for another achievement, so you can get 2 in 1 with this save file as this method will help you to go towards that. Another reason is that he starts off with a fantastic intrigue skill and sometimes a partly filled intrigue lifestyle that gives extra bonuses for plotting murders (which obviously helps but is not vital as I did this without)
The main reason is the context for the 1066 start. There are 3 independent Kingdoms in Northern Iberia (Castile, Leon and Galicia) who are all owned by brothers with full claims on the others titles (Sancho II, Alfonso VI and Garcia II respectively). None of the brothers are married and none have children, meaning that if one king dies while this is the situation, the crown will pass to the next oldest brother. Sancho is the eldest and Alfonso is the middle brother, and in history when Sancho was assassinated his throne passed to Alfonso.
So to get this achievement, plot to murder Sancho and make sure he doesn't have a child. As I said this is luck based, I did it on my second try, as the first time he survived my assassination attempt and then proceeded to have a child, so I started the save again. The success chance is maxed out at 95%, but if it works you should have this achievement in around 2 in game years.