As a DM, how do I assassinate (or attempt to assassinate) one of my PCs within 5e RAW?

Question

One of my players has a detailed backstory that has him being hunted by a rival guild. I want have him be the target of an assassin, and I can handle setting up the encounter, but I'm not sure how to use 5th edition D&D rules to accomplish the actual assassination attempt.

The PC will not be helpless, nor at 0hp, so the assassin won't necessarily have advantage on the attack and isn't subject to the "Damage at 0hp" rule.

I want the attempt to be dangerous/deadly, especially since there's little chance for a perma-death. Does this have to be done with "theater of the mind" or are there any mechanics available that I haven't been able to find yet?

Answer 1

There is no simple "assassinate" action, but there are options

There is no simple mechanic for directly assassinating someone within the rules of D&D 5e, but there are a number of ways to put characters into a situation that is highly likely to result in death. First of all, any attacker can deal massive damage to a "helpless" target (specifically, unconscious or paralyzed), as described in the answers here. The simplest way to attack someone while they're unconscious is of course to wait until they go to sleep for the night. However, most people tend to wake up when you stab them, so this only gives you one good hit. If that's not enough, the assassin might consider bringing along some crawler mucus, a contact poison that paralyzes the victim for up to a full minute, giving plenty of time to dispatch them (or another suitable poison chosen from the list in the DMG). If the rival guild has access to spellcasters, there are of course spells such as hold person that can paralyze a target.

If attacking the PC in their sleep is not a viable option, there are some monsters with abilities that make them suited to ambushing, such as the aptly-named Assassin. An assassin that gets the drop on a PC can make 2 poisoned shortsword attacks, both with advantage and automatic criticals on a hit, and add sneak attack to that for a total of 26d6+6 damage, assuming all attacks hit and all saves against poison are failed. If 2 or more assassins are involved in the ambush, even a high-level PC is likely going to be in for a bad time. Obviously the assassin stat block is almost perfectly crafted for the purpose of assassinating people, but you can certainly find other monsters appropriate for the task as well.

(Note that the assassin's stat block appears to be heavily based on the stat block for a 12th level assassin rogue. If you need a lower level assassin for balance purposes, you could go back to the rogue class description and build an assassin of the desired level.)

These options all give the PC a chance (as they should)

None of the above options will allow you to instantly and automatically kill the PC, which is a good thing, because player agency is important in D&D. In each case, the PC has a chance to notice the ambush before the trap is sprung, avoid the attacks, make their save against poison, or otherwise evade the trap through one or more rolls of the dice or clever use of spells like alarm, which all makes for a much more interesting and engaging experience for the players.

Answer 2

The problem with assassination is — are you gonna pull the trigger?

I think the answer by Ryan covers all the obvious points. An assassination is nothing but an ambush on the lives of the PCs, hitting them when they do not expect it, are ill prepared or already down on their luck and weak. The entire arsenal of monsters, spells, items, and poisons in the game is at your disposal for this, with the Assassin tailored for it. No need for special rules here. You very likely are competent enough to craft an encounter to suit your needs.

What I find much more difficult with assassinations is: how brutal are you going to be? If these opponents do know the player characters, they will have a good gauge on what the PCs can do, what they can survive and what it will take to take them down.

So, why would they send anything less than a TPK? And I do not want to TPK my group. So I send a squad that will maybe hurt the PCs, but subsequently will be defeated.

It always strains my suspension of disbelief when there is a powerful, organized, intelligent group of enemies that can scry on the PCs, and it sends an incompetent assassination attempt. Why? Who would do something like that? (It's another matter, of course, if they are incompetent.)

This makes assassinations either lame or unfair.

I have come up with three solutions for this dilemma:

1. Give the PCs the ability to detect the preparations, and to preempt the attempt. To size up the PCs, their typical movement patterns, their home base and so on, the enemies will have to send spies, cast scrying spells etc. Don't handwave this. Have them actually post the sentries, the same beggar every morning observing the inn and whatnot, and allow the PCs Perception or Insight checks for each one of those to realize that something is going on. This can be a creepy experience, the fear of the attack can be worse than the relief when it finally happens, and now you have a race who can better and faster seize up the other side and strike first.

2. Send them a warning. The bad guys also do not want to waste their costly resources and risk their killer being offed. Have someone shoot the PC with a badly poisoned arrow from a rooftop or afar. When we were playing Dragon Heist, and disrupted Xanathar's ops in the process, our DM had an assassin from Xanathar shoot one of us with a poisoned bolt, then vanish into the sewers. The bolt had "Greetings from Xanathar" written on it. We got the message that we better not mess with them, or find a way to arrange matters, or they would send something much worse.

3. Don't do it. This is the worst solution, but I have to admit I stooped to it and it works too. The players do not know that there is an assassination waiting for them. Nobody is forcing you to execute on it, if they are busy having fun with other adventures. This sucks if you want a believable world that has things going on without the player characters being on stage, but it is a practical solution. Just not a good one.

Answer 3

5e has plenty of rules for combat, but relies on rulings outside combat

I googled a list of medieval assassinations. In summary, they boil down to stabbing someone unexpectedly.

5e actually doesn't have rules for this. It has plenty of rules for combat - which is when aware, prepared, and actively resisting people fight each other. But otherwise nothing for stabbing someone in the back.

Instant death

In my games I run with instant death. If the party sneak up on a band of sleeping goblins and slit their throats, they die, no rolls needed on the attacks. If the rival guild manage to catch the player asleep, same thing.

Preparation is the key to successful assassination

The difficulty in assassinations is everything leading up to stabbing someone in the back. Figuring out where they sleep, when they sleep, how to get in there, then actually sneaking into the room. The final stab is easy.

Make sure to telegraph

This is great gameplay when it's PCs doing the assassinations, but when the PCs are the targets it's a lot less interesting. If the party don't know something, it functionally doesn't exist. If a PC wakes up dead one morning it doesn't make a difference to them whether the attempt was planned by the guild for months.

There needs to be clues so the party can react. Some people will say you need so many clues that the party definitely get the hint. I disagree. As you said, it's ok for the PC to die. So when the PC does die, you want the party to at least be able to look back and say "ah, this and that were clues that this was coming".