Complex sentence: They won the league in its first season, and were promoted to Div 1 Svenska Serien Västra, one of the first-tier divisions, but were relegated back to Divison 2 Sydsvenska Serien after a single season.
Split into simpler sentences: They won the league in its first season, and were promoted to Div 1 Svenska Serien Västra, one of the first-tier divisions. However, they were relegated back to Divison 2 Sydsvenska Serien after a single season.