The thing is, what do you want to achieve with the assassin? Is he there to kill a single person? Is it just a straight up fight or is it a real attempt at killing off one person?
In the past I have used the following to make assassination terrifying for the PCs.
- If there is an expendable NPC, it is simply found dead later on.
- The party is drugged. Everybody starts the encounter Unconcious (save ends) The only reason they get the save-ends rather than just all being killed in their sleep is that a cat familiar is screeching out trying to wake them.
- The combat starts as somebody stirs from their sleep due to the cat and is immediately auto-crit by the assassin.
- The assassin's dagger has a silence spell on it. Once hit, no sound will travel past 2ft away from you.
- The assassin always grabs and silences (garrotte). So the person can't call for help or reach their friends.
- The assassin can pull a bloodied person into the hells to try to finish them off 1v1. The person is taken out of play (save-ends) and has to fight the assassin solo.
- When fighting in groups, the assassins use a special ability that makes both them and the person they are grabbing invisible (but not too each other). As an extra annoyance, this ability destroys line of effect to the victim. The victim can also not see anybody except for the assassin.
The feeling of dying alone, unable to get help while in a camp surrounded by friends is pretty powerful. This really makes them fear the assassin. Personally I think that just using assassins as lurkers is pretty weak if you want the PCs to fear them.
Oh, one more thing, the wonderous item I think it is called the Ebon Skull. It puts out all light sources in a 10 square area at will I think. So see how the PCs like fighting in pitch black unable to keep a torch lit.