In the trailer, through a combination of shaky cam footage and lo-fi creepiness, we see a researcher documenting a snowy landscape within the Hisui region (where Arceus is set), only to encounter what he believes to be a wild Pokémon.
When the two ultimately meet, it ends in death. Or at least injury — the trailer cuts out before we can tell, but things don’t look good.
Maybe the researcher encountered a mythical Pokémon, leading to his untimely death. Based on the geographical location, one can assume there will be some ice-type Pokémon that should be approached with caution and not, say, right up close with a video camera.