Here is how Yuna returned to save Equestria and Skylands in Princess Yuna of Skylands.
At the Friendship Rainbow Kingdom, Yuna returned.
Princess Yuna: Mama! Aunt Celestia!
Princess Luna: Yuna! (hugged her daughter)
Princess Cadance: You came back!
Princess Celestia: We were so worried about you!
Princess Yuna: Princess Twilight? I'm so sorry I've left home, Can you ever forgive me?
Twilight Sparkle: We forgive you. You've always been mine, Sunset and Trixie's faithful student.
Princess Yuna: Where're Papa, Uncle Duck and Shining Armor?
Trixie: Captured by Bowser and his company.
Sunset Shimmer: Pythor had betrayed us and join them, He helped Bowser and Kaos gather all of the Skylanders.
Princess Yuna: How could he do this?! I thought he was our friend.
Twilight Sparkle: I'm afraid I put too much trust in Pythor because of our friendship and the effect that friendship and love would have upon him.
Trixie: With the Kronos Stone, Bowser has enough power that he now has the strength to steal the powers from the Skylanders as well. Without the Air Skylanders to control the wind, Master Eon will no longer be in Skylands. Without the Earth Skylanders to protect the earth, Earthquake will never end. Without the Fire Skylanders, Volcanoes will sprout endless lava. Without the Life Skylanders, All of the plants in Skylands will die.
Sunset Shimmer: Without the Magic Skylanders, Black Magic will never be stopped. Without the Tech Skylanders, All machines all of Skylands poses will be powerless. Without the Undead Skylanders, The Undead spirits will eventually go out of control. Without the Water Skylanders, There will be no one to control the tsunami sized waves.
Twilight Sparkle: And without the Dark and Light Skylanders, No one will control the pitch black in Skylands.
Princess Yuna: There must be a way for me to set things right.
Twilight Sparkle: There is. I'll have no choice but to give you this. (brings out the Lightcore Amulet)
Princess Yuna: What is it?
Twilight Sparkle: This is the Lightcore Amulet, It was the source that will restore the Skylanders' Power. Buzz asked me to keep it safe until there's somepony I can trust. (puts it round Yuna's neck) And now, I'm trusting you to guard and use it well, Yuna. Make sure you don't show this to anyone or anypony else unless there are friends you can trust.
Princess Yuna: Don't worry, Prncess Twilight. I won't fail you, Or myself or anyone or anypony else.
Twilight Sparkle: Good luck saving Equestria and Skylands.
Princess Yuna: Thank you.