Draconic Flute Crystal?

What are the crystals and where do they come from to have 100 of them for this?

Got it from turning in a level 25 dragon for the Academy thing.

Well as the description said, get 100 of those crystals to combine them into a flute that summons a specific dragon you can tame.
You can only get those crystals from the dragon academy boxes (by sacrificing a level 21 or higher dragon, lv11-20 doesn't net a box).
Though it's not worth going for the crystals, if you send a lv100 you get a box that contains one of these special dragons or alternatively as unhatched eggs from dragon arena monthly rewards.