During the time Arthur spent in the Ever Plane training, he met Alice the fairy, one of the only friendly faces he'd see.
Why yes my name's Arthur, I don't know about chosen however. Pleasure to meet you.
Well aren't you Arthur, the chosen one? I've heard a lot about you.
In one of his adventures, Arthur also met Astrea, the goddess of temptation.
Psssst hey you, let me show you something.
What is it?
Focused on his quest, Arthur denied the temptress.
But... My people need me, I cannot stay.
This is all yours, but only if you choose to never come back to the real world.
Later that day, Erwin told Arthur the real reason he was the chosen one. Just as his father, he carried supernatural powers, he was born with those powers.
Your father knew this day would come, he planted this tree decades ago, it reflects your family. As of now it is dying, but you must save it before it's too late, you must stop your step-brother and fulfill the prophecy.
All of a sudden Arthur's emotions turned into some powerful energy, radiating from his body.
I must return today to finally avenge his death and bring peace back to the kingdom.
Oh my... You're even more powerful than the prophecy said!
But you mustn't go, you're not ready yet.
Yet he left anyway, he went back to the kingdom in order to finally defeat his brother.