Thrones have an affiliation with a monster. Only the affiliated monster can sit on it and use it. It starts out as the king monster generated on the throne. If the monster dies, its killer becomes the new affiliated monster. This would need some flavor for why the king doesn’t just sit on it and use it until it vanishes; there are a few possible ways to handle that. One is that though the throne effects are good, the king is flavored as not wanting to lose the throne. Another is that throne rooms will start empty except for the king and maybe a couple guards, and the king does in fact use the throne’s powers against you (without risk of it vanishing since he is the original owner, or something).