Crusader Kings III

So this one isn't so hard as to be impossible. It will just take patience and luck.

As detailed in the above video you want to pick the 867 start and choose to play as Marzoban Wahsudan of Gilan. Reason being his religion allows familial marriage.

Follow what Enigmaticrose4 does in the video and with a bit of luck you will get the achievement in a couple of hours.

Just remember you must be in Ironman Mode for achievements to unlock.

This achievement is simple, if a little tedious.

In your game rules set Diplomatic Range to restricted. Pick the Duke of Iceland. Make your own ruler and give him/her the best fertility traits, pureblood, and start him/her with a spouse. Make them follow the Messalianism religion (which allows incest).

Now all you have to do is have two kids, have those two kids marry, have their two kids marry, and so on. You will get this achievement on the fifth generation (Starter character, children, grandchildren, great grandchildren, great great grandchildren). Your only worries are your vassal who might get ambitious and if you get sterile and inbred, which makes fertility -100%. Otherwise you can just speed 5 until you are playing your fifth gen character.

Another tip: Make all of your characters follow the seduction route. Especially later on when inbred traits start appearing the extra fertility will help.