How would you adjudicate an attack on a sleeping target? What skill checks would it take for an assassin to sneak up on a sleeping victim and strike?

Step 1: Does the sneaking party need to make a Stealth check? If so, is it opposed by the would-be victim's Perception? And if so, what penalties does the victim suffer for being asleep (or what bonuses does the assassin enjoy for the same reason)?

Or would the difficulty of the Stealth check be based on the terrain instead (like a creaky wooden floor would be harder to sneak on than a concrete floor)? Failure indicates the sleeping party gets to make a Perception check? Or maybe automatically wakes up?

Step 2: So, our assassin has reached his target. Do we roll for initiative—Cool for the assassin versus Vigilance for the sleeping victim? Do we upgrade or downgrade—or give boosts or setbacks—to either side due to the victim being asleep? Or does the assassin just get a free strike, as if the victim were staggered?

Step 3: Let's assume our would-be victim detects the assassin just in the nick of time. Can the victim perform any actions, or does the grogginess of having just woken up warrant the staggered status effect? Or maybe the disoriented effect?

Final question: How, if at all, would any of the above change if the sneaking character were trying to steal something off the sleeping character instead of kill him?

I think you'd need to make a Stealth check if you wanted to creep up quietly.

I think the defending character is entitled to oppose it with Perception - plenty of people are lighter sleepers (by nature or training) or have honed instincts or just plain species traits to detect this kind of intrusion.

I wouldn't roll initiative until the assassin is either noticed or makes an attack.

Disoriented might be appropriate, depending on the character. Maybe make a Discipline or Cool check to avoid that status.

I wouldn't change anything for stealing, except you would need to roll probably Skullduggery to lift an item off the sleeping target, they would be entitled to another Perception check to notice and wake up (maybe make that opposed again) and if they don't wake up then the thief makes a third Stealth check to get out of the immediate area without being noticed on his egress.

Step 1 dice pool seems simple enough. Nothing really changes because the target is sleeping, it's just a matter of adding and subtracting the usual environmental bonuses. When possible I make the PCs roll, so it would be their Stealth vs NPC Perception, with boost dice because of the sleeper and setback because of the creaky floor; or if the PCs are the sleepers, their Perception vs the NPC Stealth, with boost dice because of the floor, and setback because they're sleeping.

Success with Threat: target shifts in sleep to a disadvantageous position (apply threat to next roll as per combat chart)

Failure with Advantage: target pops awake, but looking in the wrong direction (apply advantage to next roll as per combat chart)

Agreed with Kshatriya, no need to roll Initiative if the assassin is not detected until an attack is made, so the assassin would get a free attack if undetected.

If just waking up and rolling Initiative, you could start all the wakers with, say, Disorient 4. Then they roll Initiative, and they can spend successes to reduce the number of turns they are Disoriented (1-for-1), but this also lowers their position in the sequence. So if they roll 4 success they could remove the Disorient immediately but end up with zero successes when calculating Initiative position, or spend 2 to reduce to Disorient 2, and still have 2 successes to determine position. A Triumph would remove the Disorient condition.

Stealth or automatic. Stealth if you know it's a crucial roll GM wise. If it's just a kill and not pivotal to the mission than auto kill.

I would only allow an auto-kill on a minion, barring like...a crit and a lot of Vicious/Lethal Blows.

I would only allow an auto-kill on a minion, barring like...a crit and a lot of Vicious/Lethal Blows.

Agreed, and I'd like to clarify the "free attack if undetected" above. I was assuming that the target here wakes up at the last possible second and so is able to roll or dodge or otherwise react, even if clumsily. But if the target really is still sleeping and you want to allow that situation, there's almost no point in rolling an attack, except maybe to see if the attacker fumbles. The target should basically be dead instantly. Even D&D allows for coup de grace rules to avoid situations where a 20th level barbarian can sleep through a night of kobolds stabbing him with daggers...

So for narrative purposes I'd allow the PCs or Rivals or Nemeses (whoever is the target) to blink awake as the knife comes down.

Thanks, everyone. This is really helpful (and will have echoes in the Alien Compendium I'm working on soon).

Depends. To be honest, unless such a character was intended to be a major nemesis or indeed had a sleeping pattern that would prompt them to be swift to rise, I would label it as an automatic kill as by the time the person would stir; fatal damage would already be done.

Of course there are plenty of ways to protect major NPC's otherwise, including alarms, booby traps and petrols. Other ideas include lawful investigation (if it's a unlawful killing) that may raise obligation if it happens in civalised space. Otherwise theres an additional option in that the party don't actually know where he spends his nights. Just it's like in the fiction; even the mightest warriors die if you stab them in the heart before they rise.

I'd first and foremost ask for a Stealth check to creep up on the target unnoticed. Then I'd probably let the PC kill of a minion without any further ado. For rivals I'd let them add levels of Lethal Blows according to the result on the Stealth check, and for nemeses it would be a straight-up attack roll with a boost die or three based on the Stealth check. All other talents (Quick Strike, Sorry About the Mess, etc.) would of course apply.