Soul Gems

Soul gems are special type of gem that have the dark ability to hold souls and drain magicka from them. Once the magicka is completely drained, the captured soul dissipates and most soul gems become flawed and unusable through this process. Use of soul gems in enchanting is taboo in most societies and considered worse than murder (as you are permanently destroying a being). Soul gems are of great use to necromancers. Items that are prepared with soul gems may be enchanted to draw in souls and thus have a hands-free way of recharging themselves. Umbra was a sword that did just this thing.