This story is a sequel to Daughter of the Unicorn King
After the Crystal Heart destroyed Sombra, everypony thought that was the end of it. But it wasn't. Sombra still lives and is struggling to reform himself. Cadence receives a dream vision telling her this much and an urgent message. 'Learn the truth and save him'. Now she and the Mane Six must decipher this and unearth the truth while at the same time figure out who this mare in their dreams is.
This story is an official sequel to the story posted by GhostQuill so make sure to check that out.