Complex sentence: Previously it competed in the newly created Campeonato Nacional de Seniores, a competition created in 2013 after the merging of the Second and Third Divisions (third and fourth tiers of the Portuguese football league system).
Split into simpler sentences: Previously it competed in the newly created Campeonato Nacional de Seniores. A competition created in 2013 after the merging of the Second and Third Divisions (third and fourth tier of the Portuguese football league system).