Lucasfilm has made no secret of the fact that the big villain of the upcoming Obi-Wan Kenobi series on Disney+ is Darth Vader.
Initially, Obi-Wan had a different structure and a different villain — Obi-Wan’s old prequel nemesis, Darth Maul.
According to The Hollywood Reporter, Maul was removed and replaced by Vader during the period when Lucasfilm decided to replace the show’s original writer, Hossein Amini, with Joby Harold.
But apparently he was a later addition to the process of developing the show.