These spirits are essentially are just very empty, corrupted souls. (That's as much detail as I can really give yet!) He may have more difficulty devouring them since they are not contained in a vessel and have taken form on their own, so he will need to beat them in a normal fashion without forcing them to flee. After beating them, he would be able to devour them, but not bend them to his will, as they would be in Drac's tumtum and therefore not able to follow orders.