This is a short three minute ONA for The Ancient Magus' Bride. The ONA provides a sneak peak of what to expect from the anime after the first season. It does so in the same style as the main anime with pretty good animation, good music and a nice amount of fairytale mystery. A problem with this ONA is that it only seems to be available in Japanese, however during the entire ONA there's only four sentences spoken by the characters with the rest of the ONA having background music instead. The translation of the four sentences can be found in the episode discussion onMAL so the language barrier is pretty low.

Overall this is a pretty neat teaser/trailer for what to expect from the anime moving forward. You can watch it if you're curious but since the animation and sound are both pretty good you can also just consider it extra content for the anime.