A soul crystal is a magical artifact used by the Order of the Flame to bond the soul of a human with that of a dragon.
Later on, Rynn and Arokh encounter a soul shadow, that of a fallen Knight of the Order, Tiuri. In life, she was the guardian of the city of Alwarren. However, when the barrier around Alwarren went up, her dragon - Tagorah - was trapped inside while she remained outside. The war giant, Tultal, killed her dragon. She faded away thereafter, forever fettered in the outskirts of Alwarren, unable to pass on. Rynn recovers Tiuri's soul crystal and lays it with her bones. This sets her spirit free to go join Tagorah in the afterlife.
Drakan: Order of the Flame - Drakan: The Ancients' Gates