Here is how the heroes meet with Morro and the Redeemed Vermillion in The Overlord Returns.
Soon, they arrived at the Master Temple of Ninjago with Wu, Garmadon, Ray and Maya.
Sensei Wu: We'll be safer here for now.
Sensei Garmadon: While we work on a new strategy to stop the Overlord and Lord Vortech, we must train in order to save Equestria along with the LEGO Dimensions at the same time.
King Lagravis: And we shall, Garmadon.
Laval: Are you sure you're okay, Li'Ella?
Li'Ella: I'm sure, Laval. Don't worry about me.
Ray: We'll take care of her for now, Laval.
Maya: And don't worry, Tormak. We won't let anything happen to her.
Tormak: Thank you, Maya.
So, Maya and Misako kept Li'Ella in good company.
Just then, Morro appeared out of nowhere in spirit as he became in all flesh in blood.
Morro: Hello, Wu.
Sensei Wu: Morro, what're you doing here?
Morro: No cause for alarm, Wu. I have come to help you.
Sensei Garmadon: But why?
Ray: Why helping us on our time of need, Morro?
Morro: Because I have brought some allies to help us.
With that said, Sensei Yang and the Redeemed Vermillion Commanders, Machia, Raggmunk and Blunck came up.
Sensei Yang: Hello, Cole.
Cole: Sensei Yang.
Machia: Long time no see, Ninjas.
Raggmunk: Did you miss us?
Blunck: We sure missed you.
Jay: What're those three doing here?
Neuro: It's okay, Jay. They're with us, they're betrayed by Acronix and Krux.
Lloyd Garmadon: It's the Elemental Masters, good to see you guys again.
Griffin Turner: Likewise, Lloyd.
X-PO: Looks like we got us some new allies to help us save the LEGO Dimensions.
Flash Magnus: I couldn't agree more, X-PO.
So, they formed each others' alliance in order to defeat Lord Vortech and the Overlord.