Board Thread:Questions and Answers/@comment-27020644-20161029024013/@comment-4548390-20161029024931

It means they become what they truly are inside. Like, if one is corrupted and transformed into a demon (like the Bamf Dragon of the Inferno Secret Wars tie-in) and someone else uses the Soul Gem on them, they turn back to what they once were (again using the Bamf Dragon as an example, he would return to his natural state, that of Nightcrawler). I might be wrong because I never actually saw it in action, but that's what I understood from what I read it was capable of.