Yeah, I wouldn't have a surprise round. I'd do something that involves the PCs needing to spend time either dealing with the combat or the assassination.
Suppose as an example, the Elven Assassin has some evil desert snake that paralyzes anything it bites, then it goes for the ear ala Wrath of Khan. Give it a couple of rounds and the person dies, their brain eaten. For bonus points, it has camouflage, so it is hard to see and it can be specially treated to act almost as if it is an arrow...
i.e. you don't see the initial shot, you have a hard perception check to make to see it - most of the time, people just see the person convulsing and then blood coming out the nose. But the PCs know better because they saw the actual shot where the 'good' NPCs just saw the outcome - there's a guy with a bow, he just fired at the king, but what the heck happened?
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Okay, so right before the start of the combat is the Assassin firing the snake and hitting the king. The PCs see this. They now have a couple of problems:
Convince the NPCs to be aware there's a snake that needs to be dead. And it might look like they're attacking the king, but really, *really*, they're not. And that snake is a clock.
The various Assassins trying to make sure the job gets finished. The PCs don't deal with them, it might be it anyway.
I like to give minions 5+1 hp per level. Magic Stones will still probably kill them a good 75% of the time on a hit, but it won't be automatic. Magic Stones is a top-tier at-will in the game for a reason
Suppose as an example, the Elven Assassin has some evil desert snake that paralyzes anything it bites, then it goes for the ear ala Wrath of Khan. Give it a couple of rounds and the person dies, their brain eaten. For bonus points, it has camouflage, so it is hard to see and it can be specially treated to act almost as if it is an arrow...
i.e. you don't see the initial shot, you have a hard perception check to make to see it - most of the time, people just see the person convulsing and then blood coming out the nose. But the PCs know better because they saw the actual shot where the 'good' NPCs just saw the outcome - there's a guy with a bow, he just fired at the king, but what the heck happened?
----
Okay, so right before the start of the combat is the Assassin firing the snake and hitting the king. The PCs see this. They now have a couple of problems:
Convince the NPCs to be aware there's a snake that needs to be dead. And it might look like they're attacking the king, but really, *really*, they're not. And that snake is a clock.
The various Assassins trying to make sure the job gets finished. The PCs don't deal with them, it might be it anyway.
I like to give minions 5+1 hp per level. Magic Stones will still probably kill them a good 75% of the time on a hit, but it won't be automatic. Magic Stones is a top-tier at-will in the game for a reason