Navani could travel into Shadesmar—and everything in the Physical Realm reflected there. Human beings manifested as floating candle flames. Spren manifested as larger, or more complete versions of what was seen in the Physical Realm. Soulcasters manifested as small unresponsive spren, hovering with their eyes closed. So the Soulcasters did have a captured spren. A Radiant spren, judging by their shape. Intelligent, rather than the more animal-like spren captured to power normal fabrials.
