('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.
