As I stated in my comment to the original question, I am about to betray my party. I will share the preparation that's been done and the plan for the actual betrayal now, and once the actual session has come and gone I will edit to share my group's experiences.
First off, let me say that by the criteria defined in the question, this betrayal will definitely be considered a success. There's no doubt in my mind.
And now, the plan...
The Reason
My group has been together for 14 years. Every so often we change GM's--the old GM will bring in a new (or old) character, and the new GM's previous character either is killed or takes off for story-based purposes. This is one of those times.
This is also our first stint with D&D 4e, so as we graduated from the Heroic Tier I started to get bored with my character, not realizing how dull a pacifist cleric would end up being.
I had brought up to the group that I wanted to bring in a new character, and everyone agreed that we could make it work. The next day, the next GM in line pulled me aside. He told me that as an adventure hook for his campaign, I should bring in a "fake" character and earn the party's trust. Then at the right moment, betray them and give them a reason to travel to the Underdark where the next adventure is set.
This also sets the framework for my "real" character's introduction to the party. He is a Drow, and would obviously not be immediately accepted by the group, as they are all good characters. So we decided together than my "real" character would disrupt the betrayal by my "fake" character, and save the party, giving him at least a moment to explain himself before being cut down once he comes face to face with the PC's.
The two characters are brothers. My "real" one has escaped the Drow society and turned good, though he still has a hunger to exact vengeance on his House, who put him through torture and killed people he loved. My "fake" PC is on a mission to track this renegade down and bring him back to be sacrificed to Lloth the Spider Queen, as well as any other notable people he encounters.
The Setup
My pacifist cleric was killed during a battle with a dragon. When the party attempted to raise him, he came back, but only for a brief moment. He sat up, looked the party's leader in the eyes, and said "I am at the Raven Queen's side to assist her in the defense of her citadel (foreshadowing the Death's Reach epic level campaign). Beware the Dark Elf." He then fell back to death.
My "fake" character then approached the party. He is a Drow Psion, but uses magic to disguise himself as an elf. He claims to have been sent by his High Cleric to assist the party in their current mission, in expectation that they will help him hunt down a renegade Drow assassin (my "real" character) as repayment.
This happened about 5 months ago. The party was wary of me at first. I continually changed my appearance from elf, to human, to eladrin, etc, to keep them on their toes. (Oddly enough, nobody ever asked me what race I really was, but oh well) The party started to get a little more nervous about me as I seemed to know an awful lot about the few Drow we encountered during the current adventure. I even volunteered to "disguise" myself as a Drow to try to negotiate our way out of a combat.
As the days went on, during each rest we took I would gradually mess with the player's heads. My character has the ability to alter memories, and combined with his expert disguises, he started imparting false visions on the PC's. After each session that we ended with a rest, I would send an email to a single player. I gave them a vision of my Pacifist Cleric, telling them something big was coming, something important, something they needed to watch out for. This continued for a while. During each vision I would somehow hint that the information was for them alone, that they couldn't trust the others.
Last session, we just rested before our last battle with the BBEG. I sent individual emails to each player, warning them not to trust each other, that their friends would betray them. This was mostly just to get them nervous.
The Betrayal
There are a few things that should happen soon. When we switch GM's, and the old one brings in his new character, the party will immediately not trust him (some of the visions hinted at new faces being dangerous). While this could be discouraging to that player for a little while, the betrayal takes place shortly thereafter and they will realize the true source of the visions.
On the first night is when the plan kicks into action. My "fake" character will sit down with the PC's over the evening meal, explaining to them more about this Drow he is supposedly hunting down. Beforehand, the meal was poisoned with a tincture that causes deep sleep for one hour. They can only be woken by being attacked or shaken violently.
Well, friends. I have assisted you in summary_of_mission_we_just_completed. Now it is time for me to ask a favor of you. As I've mentioned before, I am hunting a powerful Drow assassin. He is famous for crimes committed against my people. He has killed members of my own family in cold blood, and escaped without a trace.
(pass note to a player saying that he sees another PC's amulet of poison resistance turn from black to white)
The task I request of you is simple. All you need to do to help me destroy my brother... (My character turns into a Drow as he says those words) ...is lie back and sleep. Sleep so that you may be dragged into the pit and sacrificed for the Glory of the Spider Queen!
The GM then rolls the poison attacks to knock everyone unconscious. My allies--3 Driders and 6 Drow--arrive. During the first few minutes, the PC's weapons are taken and placed in a wagon. The strength-based PC's have their hands and feet shackled. The others are bound by rope. Spellcasters and those who have teleportation abilities are blindfolded and gagged. The plan is for the Driders to carry the sleeping PC's to a nearby entrance to the Underdark.
However, as the first Drider leaves the camp, my "real" character attacks. He strikes out at the Drider, causing it to drop one of the PC's it was carrying. That PC is handed a note describing what he sees:
You are violently awakened as your shoulders slam into the dirt after being dropped from some height. You are on your back. Your hands are clasped behind you in iron shackles, and you feel them digging into your ankles as well.
Above, you can see you are still outside, but night has fallen. Small glints of pale moonlight reflect off of eight huge insect legs standing over you. A truly terrifying Drow body rises from the spider's center, staring in shock at you with hatred in it's eyes. Slung over one deformed shoulder is an unconscious humanoid.
Roll Initiative. You are restrained by the iron bands around your hands and feet. You are still wearing armor. You are prone in the square of the creature standing over you. Your weapon is nowhere to be seen.
Play continues from this point with an instant message session open with me and the GM so I can tell him which character my "real" PC wakes up each round. The level of the encounter is actually pretty low, since the real dangers are waking up without your weapon, in the dark, with your hands and feet tied together.
The GM will declare what my "real" character does on each round, so that he is perceived as an NPC. Only once the combat is resolved will I reveal that it is my new character.
So per the criteria above, this really has no choice than to be a success. The adventure continues, and nobody is going to cry or leave. Hopefully everyone has an engaging encounter of panic, confusion, and satisfied rage as they eventually drop my "fake" PC to the dirt.
