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?
There's two obvious loopholes in the spell that don't require any DM fuckery.
The first is that the soul of the target must be "free and willing".
The willing part is very important. Who says anyone wants to be brought back? There's a bunch of afterlife options in d&d, but many of those are quite nice. If a soul, no matter how its body died, is happy and loving where they are, they can just refuse the call of resurrection .
The other canon option is about the "free" part. There are things (demons, and others) that can entrap a soul. It would take fairly little for that to become known to players who might then arrange for such to happen.
That's just the spell itself, no need to have ways that the resurrection spell casting is blocked unless you want it to be other reasons.
Other answers have already covered the best options in that regard; deific intervention being the most powerful option. A god with an objection to this empire could make it difficult to impossible for anyone to be resurrected, even if it came down to making deals with gods of death and the afterlife.
I particularly liked the idea of a weapon that kills, turns the victim undead, and then kills the undead. It isn't technically going to bypass resurrection by RAW, but it isn't a stretch to do DM's option of world building to make it RAW for your setting.
But you can go the route of the spell itself being blocked. Counter magic exists. The weapon that another comment suggested for turning them undead could instead just lay a counter to resurrection. Something that uses the same mechanics as countering a spell.
Those are some good points other people haven't brought up yet. The soul entrapment can be a cool plot device used by the antagonist to up the stakes or something gods can do on a whim when someone they especially like or hate enters their realms.
And the importance of being willing to come back can be applicable in many scenarios as well. For my world, perhaps it gives a sense of enlightenment, perspective, and makes problems seem smaller, like when people look at the Earth from space (to make the after life even more tempting even to people who have families and unfinished business). Or it might not be necessary if the world already sucks and the after life is nice lol.
I love that bit, about it being like seeing earth from space. I can see that being a really great scene when the players eventually figure out that part, if it gets used. Done right, it could be one of those lifetime memories for the group.