With a proper word and hand positioning, an equal number (of souls to bodies within range) will be released, fighting for possession of the mind. At 5th level and below there is no saving throw. The old inhabitants are drawn into the gem, at the very end of the line of cells.
6th->10th level, ST's at -3.
11th -> 13th, -2.
14th up, -1.
Any person can be possessed by any level of character higher than the possessee's level, but a high-level victim can only be possessed by someone at least his level. Galap-Dreidel and Teuer-Kan can take the Gem's soul-sucking attempts with no effect. If anyone manages to touch the Soul Gem, it will not attack them. But if it is then released it will immediately begin the attack, beginning with who held it last. spints too it only attacks souls thought. Whoever picks up the Soul Gem will feel an empathy from it. First, a feeling that you are holding an incredibly viscous, savage animal in your hands, and it is tolerating you. After one round of holding, holder will become aware of a growing hunger from the thing, like it's just waiting for an opportunity, looking at you and considering. Next round, a definite hunger and a silent, slimy drooling. This and subsequent rounds will be feelings of threat, want, desire, need, and hunger aimed at the possessor. Very uncomfortable, bothering thoughts. After one turn, they will fade somewhat into the background. After one turn (also), possessor will realize that gem is unable to take the soul while it's being held, but as time goes on, it will begin to fuck on owner, trying to get him to release gem.
Any attempt to get rid of it will cause a pulse of excitement and mounting tension to come from the gem.
The Soul Gem has an avoidance spell (so no one can touch and it can keep attacking) and a sequester spell permanently on it.
A vital component for release of souls is an available body. Up to five at a time is possible. But to create the two million strong army of Set, a huge event must happen.