Using a doppleganger as an antagonist....

[mikerand!]

I could use some help with designing encounters with a doppleganger, basically him / her / them masquerading as important NPC's or if they learn the PC's are a threat how they move in close. Maybe just a few examples of what people they would impersonate and how they may trick the party. Ultimately, they might try to kill one of the PC's to take their spot... Not entirely sure how that would go over, but I would like the possibility for that to exist.

Oh and I should mention we are in Ravenloft and this doppleganger is a dread doppleganger. Not a darklord or anything. There is a sizable town nearby that the doppleganger has infiltrated was part of my premise, for the PC's to investigate that.

[Footman]

That Doppleganger is probably a smart one, so why the hell would he take risk to infiltrate this merry Band of Murder Hobos?
A Doppleganger would kill a Person of Power, and take over his Place, after he has done that he would make sure to strengthen his grip on the Town.

Now if these wandering Adventurers are a threat, there are a Number of Ways to deal with them:
1. Invite them into Town, and give them a Quest, after all you have heard of thier exploits, so this is the perfect opportunity to send these Fools into sure Death. Just give them a Mission that leads into a very Dangerous territory and promise them a Reward. They will dispose of themselvs.

2. Another option would be, to frame them for a Crime they did not committ, while they are out in the Wilds, (keep your Scouts near by), make the People of the Towns nearby hate and Fear them, and make the Townspeople hunt them.

3. Make them your Pawns. Invite them, throw them a Big Feast and welcome them as Heroes. Then spin them a sob Story, about any of your Enemies, and make them kill your Enemies for you. If they die on the Job, great. If these fools kill all your Rivals for you, even better! If you can find out what they want you can make them dance like your puppets even more. They are looking for a Way out of Ravenloft? You know just the guy! Of course they need to do you a few Favors before you tell them. And if by some means, any of your Enemies is Good aligned, you can turn into other People to convince them that they are actually Evil.

[DrMotives]

Have you seen "Hour of the Knife"? It was a 2e Ravenloft module all about doppelgangers, also a fantasy adaptation of the Jack the Ripper murders. There's lots of great fluff and characterization that you can use, even if adapting the whole thing to 3.x would be too much.

[DeTess]

Re: killing and replacing a player.

First of all, be very careful with designing this attempt. Most players will complain if it's something along the lines of 'dc20 int save or you're dead.' Either have this occur as a consequence of their own stupidity, or give them a number if chances to get away.

Second, do it in secret from the rest of the group. Take the relevant player apart and resolve the encounter. If the campaign/player actions allow it, do this for other occasions too so that the players aren't overly paranoid.

Third, if you're sucessfull, let the player continue playing his character as the doppelganger (and pick the target character as someone who would enjoy this).