Since it's a RWBY fanfic with 'Grimm' and 'fantasy' in the title, it could be about a parallel RWBY world where the Grimm are even more powerful and fantastical. The story might follow a character or a group of characters as they journey through this strange world filled with Grimm. They could be on a quest to find a way to stop the Grimm from overrunning their world, or perhaps they are trying to uncover the secrets behind the Grimm's existence. There could be elements of magic, mystery, and intense battles against the Grimm in this fanfic.
Well, it could be about many things. It might be a fan - made story that combines the dark and menacing Grimm from RWBY with some sort of fantasy elements. Maybe it features the RWBY characters in a new adventure where they have to deal with the Grimm in a more fantastical setting than in the original show.