Ban previously asked the System if it was possible for him to use the wings of Eva, and it replied 'Yes'. However, that question was actually wrong, what he should have asked was whether it was painful or not.
Ban wanted to scream at this moment, but afraid of alerting the Mild Tempered Crocodiles, he gritted his teeth and endured the pain.
Fortunately, his gramps training regime was also quite painful and even sometimes unbearable, so he grew a little numb to it. It was also the same reason why he was able to endure and grit his teeth while he cut or bit his finger for it to bleed.
Ban wanted to complain at the System at this moment but he knew that it was entirely his fault this time for not asking.
He felt something squirming or growing from his back along with the intense pain he was feeling.
After a few seconds, the pain finally stopped for good and Ban now felt something had grown on his back.