On the very night when Kereberos, the first stage saint magician vampire was surrounded and then defeated by Ismatrazel, before they met.. Kahn passed Ronin to Szayel's shadow when they stood in a warship and watched the two godly saints fight.
At that point, Kahn was already aware that the fourth stage saint swordsman and leader of this vampire clan had been using him since the very beginning. That he was using Kahn's shoulder to shoot a gun.
And he turned the young man who saved his son's life into a chess piece in his schemes before Kahn even knew it.
This left a bad taste in his mouth so Kahn wanted to use Ronin, who also had his Shadow Walk skill now which allowed the assassin class fighter to completely hide and submerge in someone's shadow without even leaking or revealing the smallest bit of your aura.
