UsageThe exact requirements if any are unknown but, one controls another's soul with possession against the victim's will. Normally, meisters and weapons synchronize their souls together to virtually become one and perform attacks simultaneously however, the Demon Blade takes charge of the unofficial union and controls the body of whose wielding it, the voice of the weapon even comes through the vitcim's mouth. While the aggressing soul is attached to the other soul, the possessor slowly sucks the life away from the other person's soul. If this continues, the person will die by having their soul devoured.
An interesting turn of events are two soul possessors fighting for control of the other, Tsubaki and Masamune Nakatsukasa were seen battling for supremacy over each other leading to a complication. The souls in question fought to control the other and results contain to a constant stalemate or death of one of them
The technique of Soul Possession can be countered by a simple albeit difficult to perform technique. Using a thrid party's Soul Wavelength, the arbitrator can removed the hold the possessing soul by injecting a force into the vitcim, this overload of power can prove too much for a single soul which is the down fall of this technique. This is dangerous as the person under the technique's influence are granted power beyond their talent. Another method is performing the Soul Possession on a soul possessor, using the move against itself locks the two users into a deadlock and the end comes down to who has the strongest will.