This is where Willy has his nightmare about Megadon and then tells the others about Megadon in Thomas and Twilight Sparkle's Adventures of Avatar: The Last Airbender - Sozin's Comet, Part 1: The Phoenix King.
[later we see Willy struggling to sleep]
[in the dream world]
Willy: Hello? Hello!
????: Ah, I knew I'd find you again William.
Willy: No. [slowly looks back and then sees Darth Megadon appear!]
Darth Megadon: [as Smaug] There you are.
Willy: How did you find me?
Darth Megadon: You really think you could escape me? I always find ways to locate you, for you do not know the power of the Dark Side, like me!
Willy: I dealt with you before and I can do it again.
Darth Megadon: You better watching your back, for I will strike upon you, and then your friends; starting with that cowardly Pegasus, will be next!
Willy: [as Luke] NO!! NOOO!!!!
Darth Megadon: Oh yes!
[back in the present]
Willy: [wakes up] AAH!! [panting] Oh man, that was intense. [he then thinks for moment] (sighs) I can't hide this from my friends forever, I have to tell them about him.
[later Willy is explaining to the team about Megadon]
Willy: And so, I've been engaging him in Lightsaber duels when he would appear and I've been fighting him since.
Thomas: So that whale you fighting with at the outer space game show place, was Darth Megadon?
Willy: Yes, it was.
Edd: So, if he returns, you'll battle him again and then kill him.
Willy: Yes, I will battle him, but I won't kill him.
[everyone is shocked at this]
Astrid: Will, you just told us that Darth Megadon is almost like Darth Vader! The very Sith Lord that took over the galaxy and beat Obi-Wan Kenobi. And who was difficult to take down, and you won't kill Megadon, even if he's like Darth Vader, are you serious!?
Samurai Jack: But why, Willy?
Cadance: What could possible give you a reason not to kill him?
Zoe Trent: Willy, I don't wanna be rude, but I demand you tell us why you won't kill him!
Willy: [sighs] I won't kill him, because...
Penny Ling: Because what?
Willy: [tears up] I won't kill him, because, Darth Megadon.... is my grandfather.
[everyone is then extremely shocked]
Russell Ferguson: He's your, WHAT?!
Sunil Nevla: AAAAH!!!
Fluttershy: YOUR GRANDFATHER!?!
Edward: I don't believe it!
Sunil Nevla: Me too!
Penny Ling: Does Dolphy know about this?
Willy: No, I haven't told him.
Penny Ling: Why?
Willy: Because I don't want him to know about him.
Zoe Trent: When did you find out that Megadon was your grandpa?
Willy: Well it happened during the second duel with him.
[Megadon then swings his saber as Willy jumps down and force grabs his saber and they continue their duel]
[They clash their sabers in diffrent angles]
[as they do they start climbing up a gigantic ice berg]
Darth Megadon: [snarling]
[soon they are close to the top of it]
Darth Megadon: You are beaten, it is useless to resist. Don't let yourself be destroyed.
[they continue clashing their sabers]
Willy: I won't let that happen to myself ever!
[they continue clashing their sabers then Megadon cuts off Willy's fin!]
Darth Megadon: There is no escape. Don't make me destroy you.
[Willy starts backing up to the edge of the ice berg]
Darth Megadon: (deactivates his saber) William, you don't yet realize your importance. And you have only to begun, to discover your power. Join me, and I will complete your training, with our combined strength, we can end the destructive conflict and bring order to the galaxy.
Willy: I'll never join you!
Drat Megadon: If only you knew the power of the Darkside! Your father never told you what happened to your grandfather, has he?
Willy: I already know enough. You killed my mother and grandparents!
Darth Megadon: No, I am your grandfather.
Willy: [shocked about this] No! No! It's not true! It can't be true!
Darth Megadon: Search your feeelings, you know it to be true.
Willy: No.. No.. NOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Darth Megadon: Willaim, you can stop this conflict. Join me and together we can rule the galaxy.
Willy: [looks down at the huge drop before him]
Darth Megadon: Come with me, it's the only way.
[Willy jumps from the iceberg and falls down and down then lands with a thump on the ice, in which the impact knocks him out as the ice starts to crack below him]
Sunil Nevla: Wait a minute, if Megadon cut off your fin, then why do you still have it?
Willy: King Triton used his trident to fix it.
Edd: Ah, that makes sense.
Henry: But Willy, why would you keep such a thing a secret from us?
Twilight: Yeah, we're your friends!
Willy: I kept it a secret because I didn't want you to get hurt.
Applejack: Didn't want us ta' git' hurt? What do ya' mean?
Willy: I didn't want Megadon to try and use you guys to try and get to me, hence why I kept it secret.
Twilight: I can see your point.
Willy: (sighs) Since Megadon is here again, I'll have to go confront him again.
Fluttershy: What? Well, then I'm going with you!
Willy: Fluttershy, I can't believe I'm saying this, but I afraid I can't have you come with me on this. The same applies to Sunil and Penny.
Sunil Nevla: Why?
Willy: I'm the one Megadon wants, and he's well skilled in Jedi Arts. But that's not the big thing you have to watch out for, it's his Lighterangs.
Joy: Lighterangs? What are those?
Willy: Lightsaber boomerangs. Megadon could easily disarm someone with those things.
Willy: I better get one of the ships ready. I'll be heading out very soon.
Willy: Megadon knows where I am, and there's no doubt that he'll come for me, so I have to go to him. Plus, I sense a strong disturbinence in the Force.
Fluttershy: Will, please don't go!
Willy: I'm sorry. I have to.
[but as Willy starts off again, Fluttershy grabs his tail]
Fluttershy: Please, Will! I don't want you to get hurt!
Willy: Fluttershy, knock it off! I'll be fine!
Willy: [sighs] Listen, Shy. [strokes her mane] I appreciate you being concerened for me, but I can take care of myself. And I don't want Megadon to harm you guys, I have to go.
[Fluttershy hugs Willy tightly as Willy returns it, and then he leaves the room]
Fluttershy: [sniffs] Good bye, my friend.
Twilight: Disturbence in the Force. What is he talking about?
Dolphy: [watches him leave] Disturbence in the Force, yeah right.