Board Thread:General Discussion/@comment-220.246.156.182-20140429074520/@comment-16047389-20140522151122

DragonHunterTheif62 wrote: Drakwind wrote: I may be wrong here, but, I think it's only when a black soulgem is used can a soul be sent to the Soul Cairn. Hmm, possible, but no proof. I mean, you hear the souls saying that they were fighting necromaners when they got soul trapped, but so little necromancers have black soul gems, seeing as black soul gems are the rarest soul gem. The proof is that only Black Soul Gem can store the souls of sentient beings. Every person in the Soul Cairn, assuming they didn't travel there willingly like the Dragonborn did, was sent there using a Black Soul Gem. The whole point of Malyn Varen turning Azura's Star into the Black Star was to make it suitable for storing his own soul. The reason why Necromancers aren't just running around with Black Soul Gems is because they are the most difficult to create, and are outright forbidden to use in the Mage's Guild, and even in the College of Winterhold. Currently, the only known ways to create a Black Soul Gem are by traveling to the Soul Cairn and offering a Greater/Grand Soul Gem for transfomation (a dangerous journey which many Necromancers do not return from) or by performing a specific ritual that involves placing a Grand Soul Gem into specially built altars, then Soul Trapping it during the Revenant Shade, a celestial phenomenon that occurs once a week at midnight, when the Revenant Moon passes between Nirn and Arkay (the planet). Since traveling to the Soul Cairn is dangerous, and the knowledge of the ritual may have been lost during the destruction of the Cult of Worms, it would only make sense that Black Soul Gems are extremely rare.

Additionally, when talking to Serana about the Soul Cairn, she attests to the theory that only souls used in Black Soul Gems end up there. This would explain why you only encounter the souls of people in the Soul Cairn, and not bears, wolves, giants, trolls, or any other White Soul creature. Only sentient creatures, such as Man, Mer, or Beastfolk, have Black Souls. Thus, it requires a Black Soul Gem in order to Soul Trap them.