I'm planning a campaign loosely where players have to fight enemies backed by a larger, scarier empire that frequently sends out their agents to try to assassinate them while they try to setup a new kingdom post-revolution (think the beginning of Game of Thrones where players are on the Small Council, but they're also sort of Danaerys trying to fend off the spies and assassins of the enemy kingdom's Varys).

I want there to be a lot of cloak and dagger stuff. The players will probably have to protect themselves and fellow members of the court, the monarch (whether it's a player or NPC), allied diplomats, and such from assassins while also rooting out spies. Those resulting battles, along with adventures that I'll incorporate with diplomatic missions abroad, are what will make it DnD.

But it occurred to me as I was planning the worldbuilding for this campaign that a lot of the danger of assassinations will be lost if they can be undone by resurrection magic. Then I started wondering how kings, organization leaders, criminal syndicate bosses, basically anyone important ever dies in any high fantasy DnD world. For players I can restrict their access to diamonds or whatever, but for NPC's who are rich and powerful, not sure if that makes much sense. Besides, it's okay of players have access to the magic, but I want NPCs to be threatened by it, because it adds drama and stakes to the story I'm planning. But if players have access to it, then basically no NPC around them is in danger either, and I lose a lot of the tension I was counting on.

So looking for advice on how you would solve this. Tl;dr: How would anyone important or rich die in your fantasy world from stuff that are not old age? (assuming you want a fantasy world like I do where death is a dangerous possibility)

Restrict the resurrection spells? Restrict diamonds even more so they're rare even for kings? Manipulate the religion or cosmology of your world somehow? Do something with the resurrection spells themselves, like like Matthew Mercer's optional rules? Something else?