Shadow-Born are the lost souls of people who have had their souls stripped away from them by the Winter King. They are often described as wearing long cloaks and hoods to cover their faces, but their hands are visible, revealing that they are ghostly phantoms. The Winter King can summon them out of Pandora's Box.
Shadow-Born can strip away people's shadows and leave them lifeless, an empty shell of themselves. However, Jack has the power to replenish their souls, and can look into Pandora's Box because he has no shadow.