Ummmmm... I think you need a soultrap spell and a soulgem big enough for the creature you want to soultrap(if it's the same as Morrowind have never been a soultrapper myself) then cast it on the person/creature you want to soultrap then kill it with your weapon and it should go into your soulgem if it wasn't too powerful for your soulgem.(like, you couldn't put a Land Dreugh into a petty soul gem) I think that's how it works but not sure.
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