King Yeongjo (Korea) ordered his son Crown Prince Sado to climb into and be sealed in a wooden rice chest. Sado died in 8 days due to suffocation. It is speculated the king issued the order because Sado was mentally ill, rumored to have been murdering and raping people. Others claim he was a victim of political machinations.
Knyaz Boris I of Bulgaria blinded his eldest son Vladimir and put him in dungeon, because the latter tried to bring Paganism back in the country.
Not strictly a monarchy but Constantine had his eldest son Crispus executed. The reason is disputed but the most popular theory is that Constantine was manipulated by his wife and Crispus' stepmother Fausta.
Ivan the Terrible personally killed his own heir apparent, although it was an accident. The painting of the event by Ilya Repin is one of my favorites by a Russian artist.
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