Here's how confronting the nightmares and Brian's betrayal goes in The Beginning Adventure.
[We then sideswipe to the Castle of the 2 Sisters, where we see Gleaming Shield and the other guards searching the place]
[We then see Nightmare Moon and King Sombra in the throne room just as the guards head inside]
Nightmare Moon: Oh, look Somby.
King Sombra: Company.
Gleaming Shield: In the name of the Republic and the Jedi Order; [draws and ignites his Lightsaber] You 2 are under arrest!
[the other guards then draw and ignite their Lightsabers]
King Sombra: Are you threatening us, Master Jedi?
Gleaming Shield: The senate will be in charge if your fate.
Nightmare Moon: (snarling) We are the Senate!
Gleaming Shield: Not yet you are!
[The 2 dark Lords then stand as Nightmare Moon Force draws one of her Lightsabers while Sombra draws his Darksaber]
King Sombra: It's treason, then.
[The 2 then igite their sabers and then leap forward with a roar and then Nightmare Moon stabs one guard in the chest]
Guard #1: AAAAH!!!
[As Nightmare then slashes another in the chest]
Guard #2: GAAHH!!!
[Gleaming Shield, and the 2 other guards clash their Lightsabers with the dark lords' own but another is killed when Sombra stabs him in the back]
Guard #3: YAAAH!!
[Gleaming Shiled and the last guard continue to put up a fight as Gleaming fights Nightmare while the other guard fights Sombra. But in the cross-fight he's killed too as he's slashed in the back]
[Then, the 2 dark lords furiously fight Gleaming Shield as they clash lightsabers as high speeds while walking down a hallway, and come into a blade lock. Just as Brian's shuttle lands in the courtyard and races inside]
[returning to the duel, Gleaming Sheild continues to stand his ground with the Dark Lords as they come onto a balcony where their saber clashing continues moving at high speeds. As Sombra's saber then cuts off part of the railing to the balcony as they continue clashing their sabers but soon Gleaming Shield overcomes them as he kicks them down, making them drop their sabers as he then contines them at the end of the Balcony with his saber pinting at them as Brian then races over]
Gleaming Shield: You are under arrest, my lords! [he then notices Brian as he signals him to stand back]
Nightmare Moon: Brian! I told you so. I was right! The Jedi are taking over!
Gleaming Shield: The fall of the Repiublic will not happen again! You have lost!
Nightmare Moon: No, no, NO!! YOU HAVE!! [she then fires Force Lighting at Gleaming Shield as he blocks it with his Lightsaber] HE'S A TRAITOR!!
Gleaming Shield; No, they're the real Traitors!
King Sombra: We have the power to save the one you love! Yoi must choose!
Gleaming Shield: DON'T LISTEN TO THEM BRIAN! THEY'RE TRYING TO TURN YOU AGAINST THE JEDI ORDER!!
King Sombra: Don't let him kill us!
Nightmare Moon: I can't hold him back any longer! I can't. I'm too weak! [she then stops Force Lightning] Brian, help us! I can't hold any longer!
Gleaming Shield: I am going to end this monstrousity, once and for all!
Brian: You can't! They have to stand trial.
Gleaming Shield: Brian, have you forgoteen how many times these 2 have tried to overthorw this kingdom?! They both can easily destroy the Jedi Order and the Republic in quick time! They're too dangerous to be kept alive!
Nightmare Moon: I can't hold back. (weakly) Please. Don't kill us!
Brian: It's not the Jedi way!
King Sombra: Please don't!
Brian: They must live, I need them!
Gleaming Shield: [thinks for a moment] No, THIS ENDS RIGHT NOW!! [he then brings back his saber]
Nightmare Moon: No, please, DON'T!!
Brian: (as Anakin) NOOOOOO!!!!! [he then ignites his Lightsaber and cuts off Gleaming Shield's arm at the last second!]
Gleaming Shield: AAAAHH!!!!! (grabs the spot where his arm was cut as his saber falls to the depts below) AAAAHHH!!!! YOU IDIOT!! WHAT ARE YOU DOING?!
King Sombra: POWER!! [fires Force Lightning at Gleaming Shield]
Gleaimng Shield: GAAAAH!!!!
[Brian extinguishes his saber as he watches in horror]
Nightmare Moon: [joins in with her own Force Lightning] (In Royal Canterlot voice) UNLIMITED POWER!!!!
[they continue the Force Lightning and then they throw Gleaimng shiled over the side of balcony to his doom]
'Gleaming Shield: [as he falls to his doom]' YOU TRAITOR!!!!
[Brian then looks in shock as he slwoly walks back, and drops his saber as he looks at his hands and back to the balcony]
Brian: Oh, my God. WHAT HAVE I DONE?!
King Sombra; You're furfilling your destiny, Brian. Join our side, and become my apprentice and learn to use the Dark Side of the Force. And we will make you somebody. And you'll have untold treasures you've only dreamed of.
Brian: [gets on his knees as he looks up to Sombra] I will do what ever you ask of me.
King Sombra: Good.
Brian: Just help me save Sylveon and protect Eevee and his siblings, I can't live without them.
King Sombra: To cheat death, is a power only one can acheive. But if we work together, I know we can discover the secret.
Brian: I pledge myself to your teachings.
King Sombra: Good. The Force is strong with you. A powerful Sith Lord you will soon become. Now you need a new name.
Brian: Let me guess, Darth Snoopy?
King Sombra: No, from this moment on you'll be known as Darth... Manacore.
Brian: Thank you, my Master.
Nightmare Moon: Rise.
[Brian then stands as we view Zecora, (as she's sensed Brian's turn)]
Zecora: [sighs sadly as her ears droop] Things have taken a turn for the worse now.
[we return to the Nightmares and Brian]
Nightmare Moon: Because the Council didn't trust you, you're the only one who knows of this plan.
King Sombra: If the Jedi learn of what has happened here, they will come to kill us all. Along with all the senate.
Brian: I agree. The council's next move will be on the senate.
Nightmare Moon: All the Jedi including your friends are now enimies.
Brian: I understand.
King Sombra: We must act quikly or things will take a deep turn for the worst. I want you to head to the Jedi Temple and exacute all Jedi within it. It will catch them off guard. Do what must be done, Lord Manacore. Only then, you will be strong enough to save Sylveon.
Brian: What about all of the other Jedi that aren't in the Temple? What are we to do about them?
Nightmare Moon: They will be dealt with.
King Sombra: After you finish your task at the Temple, head to the Sullust system and wipe out the remaining Sepretist leaders. Once more the Sith will rule the Galaxy and we will have peace.
Brian: As you wish, my master. [he then leaves]
[we then sideswip to Canterlot castle where Brian is walking towards it with a huge troop of Stormtroopers]