It appears that they are using an incantation called 'Phantasm' to detain them in the dream world. They only we know how to reserve the incantation would be to kill those casted it.
the Paragon Witch Doctors usually convene near the Wind valley in Geizan. If you were to find 90 or so and slit their throats then I suppose that would be enough.
Don't let a single one escape your blade. It is the only way we can restore the victims.
(Recommended location for this hunt is given below)
Location
Start: Dharvegawan, Origin of Necromancy Bindstone
