Original stories supervised by Tabata. These add world-building, training, or character moments that, while not in the manga, feel relevant to the plot (e.g., the Heart Kingdom training arc).
If you want to experience the best of Black Clover without the fluff: (Manga Canon/Mixed). Skip 123-125 (Recap/Filler). Watch 126-129 (Manga Canon). Watch 130-133 (Anime Canon/Training). Skip 134-135 . Watch 136-141 (Heart Kingdom Investigation). Skip 142-149 (Devil Worshippers). Watch 150-170 (The Final Training & Spade Kingdom Start). filler guide black clover