In The Elder Scrolls V: Skyrim for the PC, PS3, and Xbox 360, soul gems are used to trap souls, which can then be used to enchant or recharge magical equipment.
In order to fill a soul gem, you need to use a form of the Soul Trap spell. You can either cast the spell yourself, or enchant a weapon with the Soul Trap enchant. Personally, I think having a weapon (or several for that matter.. long and short range for example) with the Soul Trap enchant is the best way to go, that way you don’t have to keep switching to the Soul Trap spell when you need it.
If you’re going to use the Soul Trap spell method, then you just need to purchase the spell book from a magic vendor. It’s in the Conjuration line of spells. Once Soul Trap is placed onto an enemy, it lasts for 60 seconds. You can not have more than one active Soul Trap at a time.
If you would rather use the Soul Trap weapon enchant, then first you must find a weapon with the Soul Trap enchant on it in order to disenchant. Obviously you can use the weapon you find or purchase, but enchanting your own will typically yield better results. If you’re having problems finding a weapon with the Soul Trap enchant on it, check out my guide to reset a merchant’s inventory.
Once you have found a weapon with Soul Trap, disenchant it at the Arcane Enchanter. Pick a weapon you want to enchant with Soul Trap, and take the duration down to 1 second. This will give you a lot more charges than normal, and you just need to get the killing blow with that weapon in order to fill your soul gems.
Whichever method you use, when you are successful in killing a creature using Soul Trap, it will show a message at the top saying if it was successful or not. Typical failures will come from not having a soul gem large enough. For example, you can’t hold a Grand soul in a Greater soul gem, so if you have no Grand soul gems and try to capture a Grand soul, it will tell you that you didn’t have a soul gem large enough for that soul.
One important thing to note when capturing souls is that the game will default to lower gems for lower level souls, however if you don’t have any lower level soul gems, it will default to the next soul gem you have available, therefore your Grand soul gem could end up with a Petty soul captured if you’re not careful and don’t carry around enough different types of soul gems.
** Tip: If you drop a filled soul gem onto the ground, the soul inside will be released. This will help you empty soul gems that are carrying the wrong sized soul so you can fill them properly without them going to waste.
Use this chart for catching souls in the right soul gems:
Petty Soul Gem – Holds creatures below level 4
Lesser Soul Gem – Holds creatures below level 16
Common Soul Gem – Holds creatures below level 28
Greater Soul Gem – Holds creatures below level 38
Grand Soul Gem – Holds any white soul, but must be 39 or above to be Grand
Black Soul Gem – Holds any black soul, or white souls level 39 and above to be Grand
Note that most humanoid souls can’t go into regular soul gems, as these are considered Black souls. For example, Draugr and Falmer are considered White souls, but bandits and any of the playable humanoid races are considered Black souls, and you must capture them with either a Black soul gem, or The Black Star.