How to run an excorcism?

[Watson]

How would you run an excorcism in the game (like a person being possessed by a ghost or evil spirit)? I suppose it could be handled as a Thaumaturgical transformation on the ghost/spirit, and aiming for a "taken out" result to force it out of the physical body of the victim.

Any other ideas or thoughts?

[devonapple]

Additional considerations:
Whether to handle it like a Counterspell or a conflict/contest of wills/power.
Whether or not to allow a priest to take out the possessing influence using a contest of wills: is a spellcaster the only option?
Whether or not to allow *anyone* with enough force of personality to take out the possessing influence.
Whether to allow the exorcist to use Evocation, Thaumaturgy, or both. Is the Thaumaturgical ritual simply a spellcaster's shortcut for a longer, extended contest usually between the spirit and the possessing influence?

[Silverblaze]

Other querries to address.

If a foreign entity can be removed from someone's mind, could another consciousness? 

Can exorcisms expel direct mind control (like possession, the power)
 (not thralls or Renfields etc.)

If Holy Touch can make a BCV back off what does it do to a possessing spirit?  Can it force it out of a body if the spirit that is invading is weak enough?

Can a powerful enough exorcism/exorcist halt a Denarian possession or remove a shadow?  I'm going to bet not, but I bet 60 shift thaumaturgy spells (this was just a random large number don't read too much into it) don't get used for that too often.

If you soulgaze a possessed person who does the gaze hit?  If the 'gaze hits the invader can it be beat into submission by enough soul gazes doing enough stress/mental consequences?

Do thresholds and circles have any effect on such a "ritual"?

Lots of questions, not too many rock solid answers.

[Watson]

Thanks for the replies!

The situation is like this: We are starting our DF campaign next week and the opening scenario is supposed to involve a robbery-gone-bad. The robber killed an assistant at a pawnshop and the violent death caused the spirit of the assistant to take partial possession of the murderer. The control of the murderers body goes back and forth, and neither have full control of the body (and the “other” mind can still see and feel what is going on, like sitting quiet in the backseat…). The spirit is afraid and cannot leave the body of the murderer on his own.

The group will need some kind of way to expel/remove/save both the soul of the murdered assistant as well as rid the murderer of the possession so that he can go to jail for his crime. Doing an exorcism was just one idea, that could be handled by the group (as there is one wizard, but no priest). So other ideas are also interesting.

One question remains – if the exorcism works, what happens with the spirit once it is forced out of the body? A social conflict to convince it to “go to the other side” would perhaps be cool, but how to do this if one can’t communicate directly with the spirit (going through a medium might work, but it is not that cool)?