Land of Departure can be in the Realm of Sleep, but not like all of you are thinking. Okay, we know that Land of Departure can change into Castle Oblivion with Eraqus' Keyblade trick. But what if this was more than just a physical transformation? Instead, think of them as two sides of the same coin and they can't exist without the other. One is awake (Castle Oblivion) and the other stays in sleep (Land of Departure) until the switch occurs again. Young Xehanort could be looking for the Land of Departure in the Realm of Sleep because since you could free worlds in sleep with the Keyblade, he can switch Castle Oblivion and Land of Departure without Aqua's help, but in the Realm of Sleep. Then, he could just go to the Realm of Light, go to Land of Departure, and retrieve Ven to probably form the X-Blade again or something.

 

What do you think?

Not possible, because that's now how it works. They aren't two separate worlds. LoD was CHANGED into C.O. Just like how LoD was probably called and looked different before Eraqus changed it to fit his own needs. That's what he says to Aqua. Master's have the ability to change a world's design to fit their needs; that the Land of Departure had been around for generations before it was entrusted to him. So basically, a world known as "Land of Departure" doesn't exist anymore. Its castle and the world itself were changed into Castle Oblivion, Aqua's castle to hide sleeping Ven.

Not possible, because that's now how it works. They aren't two separate worlds. LoD was CHANGED into C.O. Just like how LoD was probably called and looked different before Eraqus changed it to fit his own needs. That's what he says to Aqua. Master's have the ability to change a world's design to fit their needs; that the Land of Departure had been around for generations before it was entrusted to him. So basically, a world known as "Land of Departure" doesn't exist anymore. Its castle and the world itself were changed into Castle Oblivion, Aqua's castle to hide sleeping Ven.

 

We don't know if Keyblade Masters even have the power to change worlds through their Keyholes and I very much doubt it. It's just Land of Departure and since this has been a home for Keyblade Masters for generations, it's likely capable Besides, this could also work into my theory, Land of Departure goes to sleep while the newly imagined Castle Oblivion thought of by Aqua is created in it's place as that specific part is changeable by the power of the Keyblade Master. We've never actually seen the full process of this happening outside the Land of Departure, so we don't even know if it specifically transformed. Also, we don't know if Eraqus even changed it for his needs, and the three chairs in the castle indicates that it's stayed in that form longer than Eraqus became a master as those were likely for Master Xehanort, Eraqus, and possibly Yen Sid when they were apprentices and such. I highly doubt he'd keep three chairs there when he's the only one there.

 

To sum up, we barely know about this transformation as is. Eraqus may say it physically transforms, but that doesn't mean it has to proceed as he plans, he just believes that is what happens. It's not the first time in KH something doesn't go completely as thought when we haven't seen the whole process. Not to mention we've never seen it transform from outside, so we don't know if it transformed. Also, hearts are very precious things, I doubt a Keyblade Master has the power to change them willy nilly whenever they feel like it. Take Ventus for example when Master Xehanort removed the darkness in his heart, Ansem, Seeker of Darkness when he opened the Door to Darkness or Ansem the Wise when he tried to contain Kingdom Hearts.