So you've usurped your noble brother/half-brother's throne and your control is complete. Seems like the thing to do is to kill him and bury the corpse in a pauper's grave. But you're a nice Evil Overlord, so you opt simply to throw your brother, the one person who could cast your authority to rule your entire empire into doubt, into prison (or a monastery) and forget to tell anyone that he's your brother. Sometimes this is justified on the grounds that shedding Royal Blood has side-effects, or that the land will suffer if the rightful king dies. The essential criteria for this Plot trope: