The legendary birds will appear in a few chapters later, when? Soon. The reason that i'm going after one legendary  after the other is cause i have no idea how to add Y/n to the episode, so i hope you don't mind.

Y/n was currently in class, all of his classmates were there except for Ash.

Ash came in a few minutes later, complaining about his lunch being stolen by a Litten.

Turn's out that Litten has been taking food from many other's, but the people doesn't seem to mind, well all but Ash anyway.

As Ash continues to rant about his sandwich,  Professor Kukui came in to begin the class.

After class was over, Y/n received a call from his parents saying they need him in Unova and they will be sending one of their Pokemon to pick him up once he reached there .

After telling the professor about the call from his parents before going for the airport, the ride took a few hours.

Once he reached Mistralton city, his mother's Altaria was waiting for him.

Getting on, Y/n was brought to the twist mountain, a place where only the most skilled trainers can be found deep inside it. There also rumors of a very powerful Pokemon that sleeps there, but this is yet to be confirmed.

After Altaria landed at the mountain's entrance, Y/n met up his parents along with the Sinnoh champion Cynthia.

Cynthia was interested of Y/n, as she along with the other champions, heard of  Y/n luck with legendaries.

When she heard that he caught the legendary golems at Unova's underground ruins, she knew that the rumors of a Regigigas that sleeps deep in twist mountain might be true.

Traveling through the mountain took about two hours, as the insides is like a natural maze.

After traveling deep within  twist mountain, they managed to find the colossal Pokemon in all it's glory.

Unsurprisingly, Regigigas was still in a very deep slumber, to anyone else the unmoving form of the colossal Pokemon is nothing more then a statue.

 "Incredible... To think the rumors were true, a Regigigas does slumber here in twist mountain." Cynthia said in awe.

Shaking off her amazement, she turned to Y/n.

"Y/n. Call out your Regirock, Regice and Registeel. If legends are true, they should be able to awaken Regigigas."

"Your sure? Something tells me that might not be a good idea." Y/n said having a bad feeling about Cynthia's plan's of awakening Regigigas

"I'm sure." Cynthia said with a serious look.

"I hope you know what we're up against here Cynthia." Y/n said while pulling out his heavy balls. Y/n is also glad that his parents didn't come with them, if things went haywire in here they will be out of the danger zone.

Y/n called the out Regis, the trio stood a few feet from Regigigas. The "Dots" on Regiggias "beeped" one by one before they along Regigigas's six eyes glow. The colossal Pokemon started to move, dust that have collected over the years  fell off, Regigigas slowly started to move forward.

The colossal Pokemon stopped infront of the Regi trio and slowly lifted it arm.

Before the human duo can react, swung it's arm with great force, knocking away the legendary golems in one move.

  Y/n tried to order an attack, but Regigigas used confuse ray to control Y/n's legendary titans.

The titans who were once loyal to their trainer attacked him with hyper beam, which said trainer dodged by jumping out of the way.

The four started to rampage, firing attacks left and right.

The human duo jumped out of the rampaging legends way as they made their way to the exit.

" Oh no! If they keep going they'll reach route 7!" Y/n said in alarm.

 "if we don't stop them it will be a disaster." Cynthia said gravely. " It's like just the legends said, Regirock, Regice and Registeel will become the three pillars that will protect Regigigas."

The duo got on Cynthia's Garchomp to catch up with the rampaging legends, who were half way to route 7.

"Alright you three, return now!" y/n called out trying to recall them. But the beams from the heavy balls just bounced of them, its like trying to recall a Pokemon that doesn't belong to him.

"It seems that we need to weaken Regigigas to set your Pokemon free." Cynthia said.

Y/n nodded. Sending out his Rayquaza to try to weaken and calm down Regigigas.

"Use stone edge!" Both trainers called out. Their dragon types sent many sharp stones at the four, but was blocked by the titans hyper beam.

"Darn it, i'm sorry you three but this is for your own good!" Y/n thought before calling out. " Earthquake!" 

Rayquaza slammed his tail to the ground, causing it to shake violently and damaging them. This caused Regigigas to be weaken enough for Y/n's titans to snap out of Regigigas's control.

  Regigigas roared ,trying to fire a hyper beam at them was to weak to do so. Y/n used his aura is see if he can do anything to calm Regigigas down without fighting it.

 Y/n sensed Regigigas's emotions, Y/n felt Regigigas was angry for being awoken from it's slumber and being attacked.

Y/n could also fell that Regigigas was pleased? After thinking for a minute, Y/n then realised that it was pleased by his creations, his children have a strong trainer to look after them.

Almost sighing, when Garchomp landed he walked to his titan's.

 "Would you like to stay with Regigigas?" Was Y/n's question to them.

The three looked that each other at that, knowing their trainer will release them if they agreed. But if they do agree, they'll be resealed at the underground ruins .  While their loyal to their master and creator but their also loyal to their trainer.

After a few minutes of what Y/n thinks is them mumbling to each other the three made their decision, Registeel step forward and told Y/n about their decision.

" You three are serious about this?" Y/n said with wide eyes.

The legendary titans nodded, seeing it as a win win for them.

" But don't you need to discuss this with Regigigas as well?" Y/n asked

"What did they say?" Cynthia asked curious .

" They want me to catch Regigigas." Causing her eyes to widen. " Cause in their words it's a win win situation. They get to stay by Regigigas's side while still staying by my side." 

 " I'll admit i'm impressed by your Pokemon loyalty to you." Cynthia said with a smile at the loyalty Y/n's Pokemon were showing him.

 "Well you three, don't forget it also Regigigas's choice as well." Y/n said seriously.

Regigigas beeped something that caused Y/n to sigh, after telling Cynthia about the colossal Pokemon decision with stay with it's creations and children, Y/n tapped a empty heavy ball on Regigigas, it didn't put up any fight so Regigigas was caught.

  Soon few Pokemon rangers came to ask the two some questions about what happened, one of them was Y/n younger twin brother B/n.

The two were very alike, but with somethings different form each other, While Y/n's has H/c hair ,B/n had red color hair. And B/n eyes were purple that Y/n's e/c,  also their uniforms are different as well.  

The two share many interests but they both chose different paths, Y/n walk the path of a trainer while B/n chose the path of a ranger.

 After things were calming down, it was getting let and Y/n needed to get back to Alola.

After parting ways with his brother, was about to get on the plane to Alola, Cynthia said something surprising as her parting words.

  " I guess the nickname we champions gave you fits, till out next meeting "Master of legends".

Y/n was shocked by the nickname, before he could say anything about it the Sinnoh champion was already out of sight.

 It was night by the time Y/n reached Alola, once he reached home, he told Coco and Ahsha about what happened.

The two were surprised to hear that he met the champion of Sinnoh, the three decided to go to bed as it was almost midnight. Y/n was tired, thinking about the days events as he laid on his bed, he smiled at the fact that he saw his brother again before falling asleep.

Man this took longer than i thought, my computer wasn't cooperating with me, always cutting the connection. Anyways next time the Y/n and Ash will help Lille to care for the egg Ash brought form Kanto. 

What legendary you want next? The legendary birds will show up in a few chapters, so your choices are The legendary beasts, the forces of nature or  the swords of justice. You have 3 chapters to choose.