the Jester
Legend
Your players are likely to feel cheated if you can't show them that you actively gave them a fair chance to prevent this from happening.
I'd give them several chances to notice that someone is trailing them via Perception (or perhaps Insight) checks. I'd also do the whole "snatch" trick strictly by the rules and on initiative. (Depending on the setup, I would probably have a final "guardians of the artifact" encounter and just stay on initiative until the snatch was resolved- probably have the npc in question attempt to snatch the artifact while the party is fighting the guardians in mid-combat.)
Be careful! This kind of thing can go awry VERY easily.
I'd give them several chances to notice that someone is trailing them via Perception (or perhaps Insight) checks. I'd also do the whole "snatch" trick strictly by the rules and on initiative. (Depending on the setup, I would probably have a final "guardians of the artifact" encounter and just stay on initiative until the snatch was resolved- probably have the npc in question attempt to snatch the artifact while the party is fighting the guardians in mid-combat.)
Be careful! This kind of thing can go awry VERY easily.