*SPOILER ALERT FOR HTTYD:RTTE (Race to The Edge)*
The Dragon Eye is a key part of the gap between the first and second movie. This is because the dragon eye had been in many Hunters hands over generations and probably the Ingermans too (Fishlegs). This was all changed when Hiccup found it on a ship the used to belong to Dagur the Deranged. This was when the conflict all started...
After a while Viggo Grimborne found out about Hiccup and his Dragon Riders. He went for them with his brother and Dagur. This proved to be challenging at first but then Hiccup triumphed every time.
Hiccup Found out how to harness the Dragon Eyes power and knowledge, this meant that he could put all the puzzle pieces together. After he had done that they found the King Of Dragons.
This is where the story ends though because Hiccup, the man that he is, didn't want the Dragon Eye or his own Dragon Eye 2.0 to fall into the wrong hands.
There's a lot to be said about where it comes from so feel free to add :)