We Can Only Give It a Try

Rean, Roan, and Celis immediately went silent. As much as they didn't want to admit it, Kentucky's words held some truth. A Soul Devil is basically all about the absorption of souls. They needed countless souls to get to the Divinity Realm. Also, there is one thing they can't escape from. At the Space-Time Realm, they must start comprehending a law, or they won't advance either. 

That said, it is very likely that the law Soul Devils need to progress through the Space-Time Realm is indeed Absorption Laws. Even if it is not, it is most likely something similar. Because Soul Devils are a mix of countless dead souls, they definitely don't have a normal Elemental Affinity since the affinities of the souls that made the Soul Devils would clash with each other.