reveal said:
So.... Knowing all this, is it unfair to have both of them, or just one, use their invisibility scrolls, sneak up to the outside of camp, study the person(s) on watch for 18 seconds (this is assuming the person(s) on watch fail their listen checks) and then sneak attack?
My PCs all have pretty good Fort saves but I can see that this could possibly lead to a TPK, even with high Fort saves and Action Points.
First off, let me say that I don't see much wrong with this scenario, assuming that the opposition is run fairly. I'm not impugning you, mind you, I'm just saying that if this assassin is lvl 20, then this is a bit over the top.
But let's back up a bit. How does this assassin know where the group is camping? There are three ideas that I can come up with off top my head.
1) They are being followed. In which case, they have a chance to notice this. Any group traveling the wilderness without someone to act as a guide/Survival Expert is begging for trouble.
2) They are in an area where there is a limited number of decent places to camp, based on the terrain. Clearings in heavy woods, for instance. In which case, any group with a shred of survival instinct would either bed down paranoid or, preferably, bed down somewhere else and live with the discomfort/inconvienience. In the first instance, the group can be easily found by process of elimination and knowledge of the terrain, but they should see this and take appropriate measures. The second instance results in a lot of tramping around looking for the group, which increaces their chance of knowing something is going on.
3) Scrying. I hate Scrying. The Will save will help, if the Scryer is of an equivilent level, but if there's multiple PC's, there's a bigger risk of the BDF (Big Dumb Fighter) blowing a save. Take my advice- our DM uses this ALL THE FREAKING TIME. It's to the point where it's just assumed that we are always being Scryed on. Anywhere. Use this level of NPC omnicience sparingly, and it's nice and dramatic. Use it often, and players might key your car on their way out.
What I'm getting at is that the scenario you outline is fine, but you seem to be taking it out of context. Your players have a chance of noticing the attack as it's happening, sure, but they should also have a chance of noticing the attack being set up.
And, personally, I've turned many "Hunt-And-Kills" into "Smash-And-Grabs" on the fly because the party just couldn't catch a break. On the other hand, I have no problem sticking it to a group that refuses to think. An encounter comes to mind where the PC's turned a peaceful negotiation with a dragon into an attempted dragonslaying. Some quick illusionary work turned a particularly low-damage attack from the BDF into somehting fatal-looking, and the dragon slumped over "dead". When the BDF went to (inevitably) collect a tooth, he was eviscerated by a remarkably healthy dragon. No Ressurection, either. This was 2nd Ed, and I ruled that a dragon's digestive tract doesn't leave behind enough to Ressurect.