This is how Our Heroes has been trapped in the Realm of Darkness in Kingdom Hearts X (Chi) Back Cover.
Laval has wake up in the Realm of Darkness
Laval: Oh... Where am I? And what is this place? Well, Whatever I am, I need to find my way out of here... Wait! Where are the others? Cragger? Eris? Worriz? Razar? Bladvic? Rogon? Gorzan? Fangar? Flinx? Where are you!?
They look around
Laval: Looks like I'm on my own now. I better go find them.
He noticed he has a Keyblade with him
Laval: Oh yeah, I have this Keyblade with me. I guess I should use this to fight. All right, here I go.
He went off to find his friends
Meanwhile
Shu wake up
Shu: Oh... What happen? And what is this place? Hm, I better get out of here. (Gasp) Where's the others? Kluke! Jiro! Marumaro! Zola! Where are you!?
He look around
Shu: Great, they're not herr. Guess I should find them.
He noticed he has a Digivice and a Sword
Shu: I can use this sword and this device with me.
He went off to find them.
Meanwhile
Happy has awakened
Happy: Ow. Where am I? And what is this place? I don't like it here, I need to find a way out. (Gasp) Wait, Where's Natsu and the others? Natsu! Lucy! Gray! Erza! Gajeel! Levy! Juvia! Lily! Carla! Mavis!
He look around
Happy: Oh no! I'm all alone now! I don't like to be alone.... Well, I guess I should find them on my own now.
He noticed he has sword with him
Happy: Good thing, I have a sword with me. That will keep me alive from something scary. Alright, here I go.
He went off to find his friend and then he has been surrounded by Shadows
Happy: (Scream) Leave me alone!
Then Shu saw Happy
Shu: Oh no!
He summons his Blue Dragon and help Happy
Shu: You need some help?
Happy: Yes! And what is that creature doing to your Shadows?
Shu: Look there's no time to explain, we have to defeat them, no matter what!
Happy: Look out!
The Shadow is gonna hit him, but Laval save them
Laval: Thank goodness, I made it here in them. Are you two alright?
Shu: Yeah, and what are you?
Happy: Are you an Exceed?
Laval: No, I'm not. I'm a Lion.
Happy: Whoa! A Lion! Cool!
Shu: So what are you doing?
Laval: Helping you, of course. That's what Heroes do.
Shu: Oh yeah.
Laval: Anyway. We have to fight them!
Shu: You're right, let's do it!
Happy: Um... Okay!
They are fighting them and they defeated them
Shu: Phew. That's all of them.
Happy: We make a great team.
Laval: We did. Oh, yeah. we didn't introduce to each other. I'm Laval, I'm from Chima.
Shu: I'm Shu from Talta Village.
Happy: And I'm Happy, I'm from Fairy Tail.
Laval: It's great to see you all. So happen to your worlds?
Shu: I don't know.
Happy: Me too.
Laval: So am I. I don't know what happen.
Happy saw a Keyblade that Laval is holding
Happy: Hey, what's that Weapon your holding?
Laval: This? It's called a Keyblade, My friend told me all about it.
Shu: A Keyblade? I never heard one before.
Laval: So What about you guys? What power do you have?
Happy: I have a magic that makes me Fly.
Shu: And I'm a Shadow Wielder that can summon Shadow, like this.
He summon his Blue Dragon
Both: Whoa!
Laval: I never seen one before. So what are you doing here?
Shu: To search for my friends.
Happy: Really? Me too.
Laval: So am I.
Shu: What happen to them?
Laval: I don't know, I don't what happened to them after I wake up. Maybe we got Separated from somewhere.
Shu: So what do we do now?
Laval: We have to go look for them together.
Happy: You're right. Let's do it!
Shu: Okay.
Then the Ground has shaking and they saw it collapsing
Laval: Run!
They ran into different paths
Shu: Oh great! We separated!
Happy: What do we do now?
Laval: Happy, can you fly across?
Happy: I can't! My Wing form has a Time Limit.
Laval: Shu?
Shu: I don't think my shadow can reach it so far. You?
Laval: I don't have anything to get across.
Shu: Great, what do we do?
Laval: Well... We should go on our own.
Happy: What? What do you mean?
Laval: It's mean we have to find our friends on our own!
Happy: What! On my own!? I can't do that!
Shu: Why not?
Happy: Because I'm too scared to go on my own!
Laval: You have to be brave, Happy!
Happy: I can't! I'm too scared!
Shu: Look Happy, I know you are scared, but you have to do this for your friends.
Happy: I know, but-
Laval: But you have to be strong, you're friends are counting you. They need your help!
Shu: You have to be strong to find them, and beside a strength of heart will make you brave, no matter what.
Laval: He's right! Everyone is counting on you to be brave, happy! Do it for your friends.
Happy: Well... Okay! I'll have to be strong to Find them!
Laval: Good! And remember what Shu said, Strength of Heart will be make you brave.
Happy: Okay!
Laval: And if we ran to each other! We'll always have to stick together as a team! No matter, where we are. We always have to find each other. And I promise I'll find you!
Shu: Me too!
Happy: I am!
Laval: Good! Good luck and be careful!
Both: We will!
They went off to find their friends