What is the prequel to Bly Manor?

I'm curious to know, could you elaborate on the prequel to Bly Manor? I understand it's a popular series, but I'm not entirely clear on the events or storyline that precede the main narrative. Perhaps you could offer a concise yet informative overview of the prequel's plot, its significance within the Bly Manor universe, and how it ties in with the main series? Additionally, are there any particular themes or motifs that are explored in the prequel that are worth noting for fans of the show?