Complex sentence: For this, he would become legendary as the main hero of the war since Johan Ludvig Runeberg wrote his epic "Döbeln at Jutas" in the Finnish National Poem "Fänrik Ståls Sägner" (written in Swedish).
Split into simpler sentences: For this, he would become legendary as the main hero of the war. His reputation was further enhanced when Johan Ludvig Runeberg wrote his epic "Döbeln at Jutas" in the Finnish National Poem "Fänrik Ståls Sägner" (in Swedish).