Skyrim:Soul Trap (effect)

Soul Trap locks the soul of a given creature or NPC in an empty soul gem immediately after its death.  The trapped soul can subsequently be used to create or recharge enchanted items.

Note that the spell doesn't automatically cause the soul to be captured. You must ensure that your victim dies before the Soul Trap effect runs out. In addition, you must also have an appropriate soul gem in which to store it. As such, weapons with soul trap can be very effective in making sure souls are captured.

If you enchant a physical weapon with the effect, it is better to use the 1 sec version of it, because attacking will renew the effect, and the killing blow will fill the gem. This will result in attacks requiring less charge.

Filling Soul Gems
In order to capture a soul, you need a Soul Gem. The gem size that is filled depends on what creature's soul is trapped. NPC souls can only be trapped in Black Soul Gems. Souls will always fill the smallest empty Soul Gem that can accommodate it. If no gem stones of the soul's level are available, it will look for progressively higher level gem stones until it finds an empty one to fill.

Now, all that is required is to find an enemy, cast Soul Trap, and kill it before the Soul Trap effect expires on the enemy. If you get the message, "There is no soul gem in your inventory capable of holding such a soul," then you don't have an empty soul gem big enough to hold it. There are no partial captures; it is all or nothing. 

Once you use a filled Soul Gem it is gone. One notable exception to this is Azura's Star, which is the equivalent of one Grand Soul Gem but is reusable. Another exception are Flawed Varla Stones , which are reusable equivalents of Common Soul Gems.

Azura's Star can also be turned into The Black Star, if you give the broken Azura's Star to Nelacar in the inn at Winterhold instead of Aranea Ienith. The Black Star acts as a Black Soul Gem that can be reused, unlike normal Black Soul Gems.

One of the easiest methods of soul trapping in large amounts is through the Soul Stealer perk with any bound weapon, as this never requires recharging and has considerable power in the early game. Mammoths provide an easy target and yield grand souls.  

If an empty Soul Gem (listed on the top) is available in your inventory, then the soul will fit into the smallest Gem available in the following way:


Enchanting
The following items use this effect.  You can learn how to enchant custom items with Soul Trap if you find one of the following items and disenchant it:
All weapons of Animus
All weapons of Binding
All weapons of Damnation
All weapons of Malediction
All weapons of Soul Snares
All weapons of Souls
Artifacts and unique items that use the effect but cannot be disenchanted include:
Mace of Molag Bal
Halldir's Staff

Scrolls/Spells/Staves
The Soul Trap ( ;  ) effect used on scrolls, spells, and staves differs from the one used for weapons. It is used for the following:
Soul Trap scrolls
Soul Trap spell
Staff of Soul Trapping
 

Similar Effects
Fiery Soul Trap

Notes
Unlike previous versions of the Elder Scrolls games, it's impossible to soul trap summoned creatures via spells, spell effects, or perks.
Whether a glitch or intentional, weapons with a soul trap effect will be resisted by corpses the player attacks.
Dragons can't be soul trapped.

Bugs
 

See Also
Enchanting: Details on using filled Soul Gems to create and recharge enchanted items
Soul Gems: Available soul gems
Souls: Soul strengths of various creatures
The Black Star: The quest in order to obtain Azura's Star