But before you can take that final step to acquire the French throne, you'll need to be independent from France itself this can be made much easier if several other dukes are attempting to declare independence at the same time. You'll need either a majority of the counties in France, or a claim on the French crown, or permission from the Pope to invade France (which will probably involve excommunicating the French king first). If you only have one top-level title, then your oldest son gets that title, and your other sons will end up as vassals to your oldest son. ('Terrible things' include assassination, putting them at the head of a far-too-small army and sending them to war, making them the court chaplain and sending them to proselytize to the pagans, and rarely-available plots to kill.) Cross your fingers and hope your sole surviving son doesn't die until after you do. Make terrible things happen to all but one of your sons.

Since you have more than one top-level title (duchy, in this case), your multiple sons will split into multiple independent realms. Problem: once you die, your sons will split up your lands.

Ck2 best succession laws