Aatrox's host- what's he capable of??

As we see from the story, Kayn uses his ult on a soldier who tried to wield Rhaast against him. If that is something that Kayn learned on his own, as well as the abillty to walk through walls, Then how is Rhaast capable of those too? MY THEORY: When a Darkin beats the host for control of the body, he somehow learns some of the things his host did, if not all the things. That being said, if that is true, what did Aatrox learn from his human host? and who was the host at the first place? Maybe there'll be answers at the gameplay rework. What do you think?