This week I have to build for my players a very strange encounter. Basically the players (in the Paizo's Rise of the Runelords campaign are on the trails of a serial killer. This week (or the next) they will be entering his manor to confront him.
The manor is sandbox-style, so I have some events scripted to make them catch up with the backstory of the manor, but mostly the party can do what he please. The main crossroads are if they go first upstairs (exploring the manor and going to the only light they can see from the outside) or downstairs, where the evil things are. If they go upstairs they will meet the ghost of the former wife of the serial killer. The ghost is evil, but his evil is directed toward her former husband and the players can chose to free her to gain a powerful ally in the upcoming battle or to fight her and rid the world of a nasty undead.
If they free her she will "possess" the body of one of them and be silent until the battle with her husband. If she survives the battle the player who hosted her will retain the ability to summon her until she dies (she cannot be healed in any way).
In addition to this one of my PC is a Pharasma (read Raven Queen if you play a standard 4E game) cleric recently rejected by his goddess and if he accept not to destroy the undead, regain the Paharasma's favor will be really hard for him, if not impossible. (Obviously he will be the hosting best candidate)
Just for the records my party is: Swordmage (4th), invoker (4th), cleric (3rd), ranger (3rd), barbarian (3rd).
Here's the Ghost:

Next in line: design the place where the PC's will meet her.