Soul Trap (Oblivion)

Soul Trap is a spell effect of the Mysticism school. It has to be cast on a creature before it dies in combat and requires an available Soul Gem large enough to contain the creature's soul. A Mysticism skill of 25 or greater is required to use this spell.

Without the required skill level there are a few other ways to obtain charged Soul Gems:
 * Use a weapon with "Soul Trap for x secs on Strike" like Umbra.
 * Use a Soul Trap scroll.
 * Buy or steal them from a local mage.

For more information on how to utilize captured souls, see Soul Gem.