A creature's soul and life force are connected, but distinct, things. The soul is basically the encoded personality, memory, and spiritual power of a creature (represented by experience points, hit dice, skill ranks, feats, class features, and so on). Life force is a special arrangement of positive or negative energy that sustains a creature's animate state. As a simile to a computer, you might think of the soul as software, the life force as electricity, and the body as hardware. Even living things without souls, such as plants, have a life force. In creatures with souls, the soul directs the lifeforce and is anchored to the body by it. If the lifeforce/body is too damaged to function, the body dies and the soul generally moves on to the afterlife after a few rounds. Some spells, like magic jar , affect both the life force and the soul together. Death effects like death knell generally attack the life force specifically, not the soul (though negative levels attack both). So when death knell succeeds, it drains the remaining life force of the creature into your body. Without any life force left, the creature dies; and without the life force there to anchor its soul to its body, its soul moves on the afterlife after a rounds as normal. Dying from that particular spell wouldn't necessarily change the course of a soul's afterlife, though it's always possible that a violent or tragic death could traumatize a soul and prevent it from resting in peace. Such souls are slightly more likely to latch onto ambient negative energy, use it to create a new life force, and spontaneously reanimate as undead, or to become petitioners in the afterlife and plead with their deities for a new corporeal form in the outer planes.