I was in a battle and my Black Kyurem was out. The other Pokémon used Hyper Voice and "killed" my Kyurem. I looked this up, normal is normally (lol) effective against a Kyurem, and it said that it was also normally effective on the type chart. I'm confused.....
The only possible explanation is that one of the three Pokémon above (Mega Altaria, Sylveon or Mega Gardevoir) used Hyper Voice as their ability (Sylveon's hidden ability) is Pixilate which changes all Normal type moves to Fairy.
That or your Kyurem was super frail, but I doubt that.
It was probably because of Pixilate.
Pixilate turns normal type moves into Fairy type moves, which are super effective against Kyurem (Dragon type). Was it a Sylveon, M-Altaria or M-Gardevoir that killed it? If yes, then it was probably Pixilate.