Murder or otherwise incapacitate your heir. Change your realm’s laws to push them out of the succession line. With a Monasticism-tenet faith, have them take the vows.
Let’s go into detail for each method below.
Easiest Way: Kill Them
There are many ways you can cause your heir to die; some being more popular than others. For example, if your heir is an adult, you can assign them to a small army then send them against a much bigger force when you’re at war. This will most likely cause them to die or go missing in action. Causing your heir to die in a war will give you no debuffs, so it’s one of the playerbase’s most utilized methods. If you’re a petty king, you can simply imprison and execute them. This will incur in tyranny, though, which is a huge setback in the early game; especially if you control a big Empire. Plus, you will gain the Kinslayer trait, which is terrible to have for starters.
If You Have Enough Prestige: Change Your Realm’s Laws
If you are on good terms with your vassals and can take the prestige hit, changing your realm’s Succession Laws is a good method to get rid of unwanted heirs. It’s also semi-permanent: as long as you have good relations with your vassals, you can always choose who you want to be your player heir. However, picking an Elective Succession while having opposed vassals can quickly cause your game to spiral into a Game Over. Adding an Elective Succession Law into your realm will cost you 1500 prestige, too, which many kings won’t be able to afford.
How does Elective Succession work?
In an Elective Succession, you and your vassals can vote for the next inheritor of the realm. The options for candidates depend on which type of Law you chose when making your realm Elective. Additionally, some decisions will turn your realm into an Elective one by default. For example, the “Restore the Holy Roman Empire” decision will grant your realm the Princely Elective law.
If You Follow a Monastic Faith: Have Your Heir Take The Vows
If you follow a Faith that has the Monasticism tenet, you can make your heir Take the Vows. This will turn them into a Monk or Nun, as well as removing them from your Succession Line. You can either do this by asking them, which they will do if their personality and relation with you allow it, or by forcing it on them. One of the most common ways to have an heir Take the Vows is by imprisoning them, then adding Take the Vows as a release condition.
Best Method To Get Rid of Unwanted Heirs
The best method to get rid of an heir you don’t want is to ask them to Take the Vows. You simply need a fairly good Intrigue skill to imprison them (and possibly fabricate a cause to imprison them, if they’re not an actual criminal). However, this method is not available for Non-Monastic Faiths. Therefore, killing them by making them into a useless army’s commander is the second best way to get rid of an heir. This will also prevent them from reproducing and staining your dynasty’s prestige. Lastly, if you don’t want or can’t do either of them, changing your realm’s laws is also an alternative. It’s a semi-permanent and playful way to get rid of unwanted heirs, but it’s not a fool-proof solution; and it can cause lots of tedious problems later on.