Board Thread:Lore Discussion/@comment-24225299-20140110165516/@comment-4705438-20140110183508

Dragon Souls are a really complicated thing. A theory goes, that Dragonborn "adds" souls to his own, creating a stronger and stronger soul. (As all dragons are likely scattered God-soul)

There is one soul gem believed to hold a dragonborn. The Red Diamond or CHIM EL-ADABAL. You may know it as the red gem in the Amulet of Kings. It's believed to hold the soul of Alessia.