This is where the foals talk to King Solar Flare in The Alliance of Berk and Equestria.
[the foals return to Equestria]
Twilight: [comes up to them] Girls, where have you been?
Nyx: Uh, nowhere.
[Twilight then goes back to her library]
Skyla: Shouldn't we have told Auntie Twilight about the village with the vikings and dragons?
Yuna: No! She cannot know, if she does, she might think it's a village planning an attack on us.
Nyx: Well, isn't there anyone we can tell?
Snowdrop: What about Grandpa? He knows a lot about Equestria history way before our parents.
Yuna: Good point.
Skyla: Let's go see him.
[soon enough, they arrive to Equinelantis]
Yuna: Now, we might get some answers about that village.
[they head inside the castle]
Yuna: Grandpa? Are you here?
King Solar Flare: Yes? [comes up] Hello, little ones. What can I do for you?
Yuna: Grandpa, we've discovered a lost village just outside the forest.
King Solar Flare: What kind?
Skyla: It's a very unique one. There are vikings, dragons, and...
King Solar Flare: Wait, did you say, "Vikings?"
Yuna: It's true. Have you heard of them?
King Solar Flare: Girls, that village is the Isle of Berk.
Nyx: The Isle of What?
King Solar Flare: The Isle of Berk. It's a village inhabited by vikings. And it once had an alliance to Equestria.
Skyla: It did?
King Solar Flare: Yes, Skyla. The Vikings and Equines were once friends to each other back then.
Yuna: How do you know so much about Berk? Were you there?
King Solar Flare: No, my ancestors were. But the story of Berk was passed down generation after generation. All the way to my father and then me.
Nyx: So why haven't we heard of Berk?
King Solar Flare: 3000 years ago, there a great war that broke out between Berk and Equestria.
King Solar Flare: It was that very war that caused the old alliance to end. In order o end the war, Emperor Rukai, the first of the Alicorns had to seal the passage way between Equestria and Berk. But after he did, he vanished.
Skyla: Oh my.
King Solar Flare: But the story of Berk still existed.
Yuna: So why doesn't Mama or Aunt Celestia know about Berk?
Skyla: Let alone my mommy and daddy?
King Solar Flare: Because I never told Cellie or Luny about Berk. I was going to when they grew into young mares but since the Draconsequuses were about to go to war on Equinelantis, I had to send them to safer passage when they were just fillies. Hence, why they don't know about Berk. And since they didn't know, Cadance wouldn't know either.
Yuna: That makes sense.
King Solar Flare: And now you've rediscovered it? Did you meet the Chief of the town?
Nyx: No, we did meet one viking though.
King Solar Flare: Who was he?
Yuna: His name is Hiccup Horrendous Haddock III
King Solar Flare: I see. I will also add that during those 3000 years, the viking and equines soon forgot about each. But there are very few who still know about it.