Just done a damage test between Gardevoir and Hydreigon.
I'm fairly new to competitive, so I may be wrong about calculations
As many people believe that Fairy Wind is a Silver Wind Variant with Fairy-Typing, I used Silver Wind. However, I set it's power to 90 due to the fact the Fairy-Type STAB would put it to that power.
Now, with Gardevoir's base 125 Sp. Attack power with no EVs or IVs, and Hydreigon's 92 Base HP and 90 Base Sp. Defense with no EVs and IVs, both with a Serious nature, the 90 powered Silver Wind is a guaranteed 2HKO, as it's power is doubled due to super effectiveness.
However, Fairy-Type is supposed to be SE on Dark-Types. For the sake of this, I changed Hydreigon into a Dark/Psychic-Type.
This makes Silver Wind (Fairy Wind) a 1HKO!
So if I did the calculations right, Fairy-Type is
not Super Effective on Dark-Types!
I put Hydreigon as a Pure Dark-Type, to represent Pure Dragon (As we can't use Fairy-Type >.<) due to Silver Wind being SE on that, and it's a 2HKO!
Using Dragon-Type and Fairy Wind in the trailer, it was shown as probably being a 2HKO!
There you have it
You may call it the 'Midnight Theory', or if I'm wrong, the 'Midnight you Mediocre Dunce! You got it all wrong! Theory'.