Here's how the Nightmare Family appears in Night Furies vs. The Nightmare Family Part 1.
[it is later in the night, but nor Yuna or Hiccup can sleep (as heroes are waiting in the shadows)]
Snoutlout: If any of those evil equines show any of their faces, I'm gonna tear them to peices.
[In Hiccup and Yuna's dreamworld]
Hiccup: Nightmare Moon!
Nightmare Moon: Who'd you expect?
Yuna: Why are you terrorizing us like this?!
Nightmare Moon: Because, I still know you from your baby age.
Nightmare Moon: I wanted you. I planned on taking you and training you to fight your own mother when you grew up, so I could take over Equestria easy!
Hiccup: You... You monster!
Nightmare Moon: I am one.
Hiccup: Why you.. GOOD ODIN!! I'VE MEET SOME BAD BEINGS IN MY TIME, BUT YOU TAKE THE CAKE!!
King Sombra: She is a Nightmare bringer after all. What do you expect?
Hiccup: AND YOU!! ENSLAVING PONIES AND TAKING OVER AN EMPIRE!!! YOU REALLY ARE A DARK STALLION!!
King Sombra: Indeed. And you're just a scawny viking.
Hiccup: Yeah? So what if I am? But I'm not like you, I never was.
King Sombra: Which is why we are after you aswell.
Nightmare Moon: AND EQUESTRIA WILL BE OURS'!!! [cackles as the dream ends]
[Then in the present]
Yuna: Did you just?
Hiccup: Yeah? You?
[then, they hear a errie sound.]
Yuna: What's that?
Hiccup: I don't know.
[they follow the sound and then they see a purple mist]
Skyla: Holy lola!
Astrid: What in Thor's name?
[everyone follows the mist and then arrive in the throne room]
James: What is that?
Princess Luna; Oh no.
[the mist then flows to the middle of the room and reveals to be Nightmare Moon and King Sombra!]
Nightmare Moon: Hello, Princess Luna.
Princess Luna: Nightmare Moon.
King Sombra: And King Sombra.
Fishlegs: Great! Now we all know each other, GET OUR OF THE THRONE ROOM!!!!
King Sombra: SILENCE, WEAKLING!!
King Sombra: Not so tough for a viking.
Nightmare Moon: And now, equestria will be ours!!
[the episode ends]