[{quoted}](name=free software,realm=NA,application-id=6kFXY1kR,discussion-id=HlEvXqKW,comment-id=,timestamp=2017-06-27T23:30:44.104+0000)
Why does Kayn/Rhaast seem to show pleasure after transformation? their VO seem to show that they work in harmony
They work together because they have to.
Kayn uses Rhaast to gain more power and Rhaast tries to overtake Kayn.
However both of these things can only work if one of them stay alive,so they have to work together.
Why can Kayn specifically either be consumed completely or overcome his weapon while Aatrox cannot?
Why is Aatrox completely consumed?
Why did Aatrox's host succumb to his blade? I strongly feel we will get an Artorias the abyss-walker story here for some reason...
You pretty much answered your question here.
Aatrox completely consumed his host already.
His host likely had the chance to fight off the darkin,but it simply failed.
It is possible that the consumation of the host is accelerated by negative emotions.
Hence why Aatrox infused Tryndamere with fury(because it'll make it easier to overtake him) and why Varus seems to succumb to his corruption(as he is full of hatred),wheras Kayn doesn't seem to be filled with negative emotions.
Alternatively Kayn might also be able to fend off the consumation simply because of his own shadow techniques.