Morrowind:Soultrap

The Soultrap spell is used to gather Souls in order to use for Enchanting.  To use it, cast the spell on the creature whose soul you wish to capture before killing it, and then kill it while the spell is still active.  You can tell the spell is active by the cloud of smoke that will circle the creature until the duration expires.  Additionally, you must also have a Soul Gem in your inventory to capture the creature in. The soul will automatically be captured in the smallest empty Soul Gem in your inventory which is large enough for that soul.  If you are successful, a note on the bottom of the screen will inform you that you have captured a soul.

See Also
Souls — a list of all creatures by soul-size.
Enchant — a detailed explanation of the process of enchanting items.
Enchanter — a list of professional enchanters who will create items for you.

Notes
You cannot capture NPC souls with this spell, only creatures.  Also, some creatures do not have souls, such as Dwemer Centurions.  Attempting to capture a soul from an NPC or a creature without a soul will cause a failure message to appear when you cast the spell.  Soultrapping NPC also has the unusual side effect of causing swirling clouds of smoke to spin around empty places in the area - sometimes very far away from where you actually cast the spell.  These are harmless, but will not go away until you save and reload.
An unlimited number of the three small soul gems (Common, Lesser, and Petty) can be purchased from the trader Fadase Selvayn in Tel Branora. The seller restocks continually: 3 Common Soul Gems, 5 Lesser Soul Gems, and 10 Petty Soul Gems.
It is possible to capture more than one soul from the same creature.  To do so, however, you must have multiple Soultrap effects.  Casting the same Soul Trap spell repeatedly will not work.  If you create multiple Soultrap spells (or a single spell with multiple iterations of the Soultrap effect) or mix spells with Soultrap enchantments or scrolls, you can capture a large number of souls -- provided you have enough Soul Gems for all of them and all of the multiple Soultrap effects are still active when the creature is killed.
The Soul Trap spell is commonly used for what is most often known as the "Soultrap Glitch", though it is known to work with many other spells besides Soultrap.  By creating a spell which has Soultrap on Target and any beneficial spell on Self, for 1 second each, and then casting the spell at the ground below you, the beneficial spell will end up being permanent.  You can do this repeatedly to increase your skills and attributes to godly levels.  Of course, almost anybody would agree that this is a blatant cheat, and an exploit of a bug in the game.
When Summoning and Soul Trapping any creature be sure to stand about six feet away if you plan to cast spells on it. Should you be too close, the game code will have the spell target you instead of the summoned creature. This bug is only in effect before the summoned creature switches from ally to enemy, which takes on average three strikes.
When enchanting weapons with Soul Trap set to Cast When Strikes, be sure to set the duration to at least 2 seconds.  A duration of 1 second is insufficient to capture a soul due to how the engine handles spell timers.
The text that displays when you capture a soul is misspelled- it says "Your have captured a soul" when it should be "You have captured a soul".
There are at least two versions of the Soul Trap spell available to be purchased from NPCs; one being On Target and another On Touch.