It appears the dusk form is a merging in the appearance of both midday and midnight forms of Lycanroc, with the build of the midday form and the mane of the midnight form, completed with the merging of the colors of both forms.
The Pokémon Company has said that “you can’t get dusk form Lycanroc on your team by ordinary methods, such as discovering the pokémon in the wild,” but it has not yet specified any details, with the likelihood of an event such as with Ash-Greninja.
In regards to the Pokémon Sun and Moon animé, in the trailer for episode 37 which airs on August 10, Ash’s very own Rockruff is going to evolve into the dusk for Lycanroc. This is very similar to how Ash-Greninja was revealed, beginning in the animé and becoming part of the games later on.
The unveiling of the dusk form Lycanroc is the biggest news we’ve had on Pokémon Ultra Sun and Ultra Moon since it was revealed in its own Pokémon Direct, a week before E3. The introduction of a new Lycanroc form might well start the momentum for other new forms, such as ultra beasts and Alolan forms. With the re-release of Pokémon Gold and Silver on September 22, there is a small expectation of some Johto pokémon getting an Alolan form.
Pokémon Ultra Sun and Ultra Moon will release on on the Nintendo 3DS worldwide on November 17.