Here is the first scene where diancie attempts to create a heart diamond, but fail, then dace will explain where to Find Xerneas in Team Robot in Pokemon: Diancie And The Cocoon Of Destruction.
(Pikachu the Movie XY)
(Opening Credits, Titles shown "Team Robot in Pokémon the Movie: Diancie and the Cocoon of Destruction", Inside The Domain, There was Several Carbink, and Diancie plays with Leonard and carbink servants)
Merrick: Please wait for us, Princess Diancie!
(Diancie Keeps Chasing from Leonard and Carbink Servants, Diancie jumps over and success, Carbink Servants And Leonard jumped over, and succeed, Merrick Almost falls and was saved)
Merrick: Princess Diancie! It's Time! Princess!
Leonard: Yoo hoo, Princess!
Diancie: (Giggles) Hide-and-seek is fun!
(Suddenly Many Carbink appeared and starts chattering)
Diancie: Shh, No! Not right now...
Merrick: Princess Diancie!
Diancie: (Giggles) You found me...
Leonard: There you are Diancie, We were worried about you playing hide and seek.
Merrick: I'm not playing, Princess. It's almost time for you to...
(Merrick Accidentally Tip over Crystal Quartz, And falling on Top of Merrick, Bort and Allotrope)
Diancie: (Gasps) Merrick! Are you all right?!
(Merrick, Bort and Allotrope breaks free from the Crystal Quartz)
Merrick: (Starts Shaking) I'm fine, thank you. (Crystal Quartz lands on Merrick's Head)
Leonard: Ouch, That's gonna leave a mark!
Merrick: (Throws a crystal Quartz) This is no time for giggling!
Several Carbink: (Laughter)
(Suddenly Several Carbink Walks Back And A Elder Carbink named Dace has Arrived)
Dace: Now, Princess Diancie, you have an important duty to perform, You must not forget that!
(the Carbinks, Leonard and Diancie arrive at the scene to see the Heart Diamond)
Dace: The Heart Diamond... For countless years, it has provided us with great energy and tranquility, But sadly, time has taken its toll on it, I know that someday the diamond will die out, and lose its sustaining shimmer.
Diancie: But look how beautifully it's sparkling now... Worrying too much will make you lose your luster.
Leonard: Luster means funny.
Dace: Please take what I'm saying seriously!
Dace: A simple yes will suffice!
Diancie: Uh Yes!
Dace: The sad truth is if the Heart Diamond were ever to be lost, our domain would be finished!
Diancie: You're exaggerating...
Leonard: Exaggerating means annoyed.
Dace: I am not exaggerating! Creating a new Heart Diamond is a task that only you can do, Princess Diancie! Before any more time is wasted, I want you to demonstrate your power!
Diancie: Okay, I'll do it. I mean I'll try... But I don't think I can.
Leonard: We're with you.
(As Diancie is using her new diamond to create, Merrick is pleased)
Dace: Yes, keep going...
Leonard: Wobbledy-Woo, Wobbledy-Wee, Make a New Diamond!
(As Diancie Try To Creating A New Heart Diamond Even Faster, She Was Struggling, The Diamonds fail to create A New Diamond and disappears, Dace Was Dismayed so to Merrick, Diancie Gives up)
Diancie: I told you. I just can't do it...
Merrick: Princess, you will one day! After all, it's what you're here for!
Leonard: Cheer up Diancie, I know many spells.
Diancie: I thank you for raising me ever since I was very young... But I'm afraid I...
Dace: Xerneas... You must go see it!
Leonard: Who's Xerneas?
Dace: That's right. Long ago, when I was journeying, there was a terrible catastrophe. This cataclysmic event occurred in a place called the Allearth Forest...
(Flashback Plays with Young Dace, Who was Trembling with Fear, Many Wild Pokémon was Running from the Destruction Smoke, A Black Tornado appears Turning The Whole Forest into stone, Dace and the other wild Pokemon began to Shiver in Fear, Seeing The destruction smoke getting closer)
Dace: (Flashback) My life force is being drained. I'm finished!
(As The Destruction Smoke gets Closer, Suddenly Xerneas Arrives, And Uses Fairy Aura, Dace Opens his Eyes, Seeing Xerneas Using Fairy aura, Protecting from the Destruction smoke, Dace Collapses into the ground and dies, However Xerneas Uses Fairy Aura To Revive Dace)
Dace: (Flashback) What? That's... Fairy Aura!
(Dace Sees Xerneas, Who Stands at the wild Pokemon, seeing whole forest was destroyed)
Dace: (Voiceover) It's Because of Xerneas that I am here today.
(Flashback Ends, when Several Carbink playing, while Diancie, Leonard, And Carbink Servants Walking)
Leonard: That's who we're going to meet Xerneas?
Dace: Yes. Leonard.
Diancie: So if I meet Xerneas and its Fairy Aura washes over me, then I'll discover my true power and be able to actually make a Heart Diamond?
Dace: You will! You have to trust Fairy Aura!
Leonard: Fairy Aura, Means trust the true power of Diamonds!
(As Diance Sees the whole Domain with Carbink playing Quartz, She Made A Determination)
Diancie: I'll do it. I'll go! I always wanted to see the outer world anyway.
Dace: You must take all of this more seriously!
Dace: What will suffice?!
Diancie: A simple yes. Yes!
Leonard: Then let's go!
(Later we cut the scene to inside a cave to outside a cave to see falling leaves, Diancie was Surprised to see the river)