Here's what I think that happened in a novelized way:
A tall young man with long and silver hair was standing on the sand, drifting his mind along with the tides of the transparent sea reflecting the clouds and the sun's rays. The young man, sat in the sand, looking down and whispering to himself "Boredom..."
xXx
"Eraqus." A deep voice was heard in the empty corridors of a light filled castle.
A tall young man with long and black hair, surprised by the voice, stood still and replied with "Yes, master?"
"It is time to monitor the island's door."
"I will get there ASAP."
With that said, the young man snapped his fingers and a pale flash of light covered the room, the young man had disappeared.
xXx
Eraqus arrived in a dark place, filled with mushrooms and a wooden door at the end of it. He lifted his right hand, which with was holding an enormous key and pointed it at the wooden door. Suddenly, a keyhole appeared. He approached the door, bended over and took a look at it. He saw nothing, exactly what he wanted to see. When he got up and turned away, the silver-haired young man was behind him.
"Who are you?" He asked
"Keyblade apprentice, Eraqus."
"Key... blade?" the young man asked in a curious tone.
"Yes."
"Have you always been in this bland place?"
"No." Eraqus said directly to the point.
"Can you come here whenever you please?"
"Correct."
The silver haired young man suddenly got an idea.
"I know your master, can you take me to him?"
With that said, Eraqus snapped his fingers and they were teleported out of the cavern. Afterwards, what we know as Birth by Sleep transcribes.