Here's how the story of the Nightmare Family goes in Night Furies vs. The Nightmare Family Part 1.
[later, everyone is in the throne room]
Snoutlout: There'd better be a good reason for us to be here at this time, Princess.
Princess Luna: There is. Nightmare Moon and King Sombra have returned.
Princess Celestia: [gasps]
Twilight: Oh no!
Astrid: Hold on, who are Nightmare Moon and King Sombra?
[the music score: "What Kind of Quest" starts playing]
Princess Luna: One thousand years ago, they took over Equeatria when Yuna was still a baby. It all started when an evil cult filled with dark and hatreed created a clone of me. And her name was Nightmare Moon.
[we then see a picture of Nightmare Moon]
Princess Luna: [narrating] "She was vowing for an eternal night". "And to be ruler of Equestria she was very powerful too, she easily beat Celestia." But things got worse when she married the Dark Stallion: King Sombra.
[we then see a picture of King Sombra]
Princess Celestia: [narrating] His heart was black as night. He once took over the Crystal Empire and inslaved all of the Crystal ponies there, as his slaves. He could even sense fear in many of the ponies. Then one day, he and Nightmare Moon crossed paths. And they fell in love.
Princess Luna: [narrating] Many dates passed until, they eventulaaly were married. Afterwards, they began to spread fear. Casting nightmares over many of the children. With help from a nightmare train, they call Cerberus. Then Nightmare Moon created an evil clone of Hiro, which she called "Nightmare Hiro." And then they got 2 bratty daughers: "Nightmare Trix and Princess Black Hole". One night, they made their attempt to take over Equestria.
Princess Celestia [narrating] That was until Luna stepped forward and challenged Nightmare Moon to a duel.
Princess Luna: [narrating] I stood for my evil self. And we both went against each other in a sword duel. It was difficult at first, but I defeated my evil self in battle. Then we used the Elements of Harmony to send the Nightmare Family ot Saturn's moon: Titan.
Yuna: So all of this happened when I was a foal?
Princess Luna: Yes. But now it seems as though they have returned.
Princess Celestia: But that's not all, they also know Jedi arts aswell.
Fishlegs: You mean they use those Light swords too?
Princess Celestia: Yes.
Princess Luna: But King Sombra uses a rare one, a Darksaber.
Yuna: A Darksaber? What kind of Lightsaber is that?
Princess Luna: A Darksaber is an anceint black-bladed lightsaber. But unlike most other lightsabers, the Darksaber's blade is nearly flat, thin, and curved to a point more typical of a metallic sword.
Princess Celestia: The core of the blade is known to draw in all surrounding light and color, leaving a faint white, electrical aura along its edges, it hummed with a higher-pitched whine than its more common lightsaber counterparts when the blade was active, and made a sound like a whistle when it was swung.
Astrid: So, it's basically like a Blackhole turned into a sword?
Princess Luna: That's one way to describe it.
Hiccup: So why am I having nightmares involving the Nightmare Family?
Princess Luna: It's probobly because you were the first viking Yuna made friends with, or maybe because you and Yuna have Night Furies. I don't know, but from what I can tell, they seem to be targeting you and Yuna.
[everyonne gasps as Nightstar and Toothless are shocked]
Yuna: What do we do?!
Yuna: Uh, Blythe?
Blythe: He said, "Couldn't we try to stand and fight?"
Princess Luna: Yes, we could. We might have a chance. Since we now have dragons on our side, specially 2 Night Furies. We might be able to advance against the Nightmare Family.
Astrid: So, what we do?
Princess Celestia: For now, we must wait and see what they'll put in tonight.
Hiccup: Alright. And if they arrive we'll be ready.
Toothless: [growls in agreement]