Complex sentence: Queen Kunti had previously conceived another son, Karna, when she had tested the mantra out--despite her protests, Surya the sun god was compelled by the mantra to impregnate her.
Split into simpler sentences: Queen Kunti had previously conceived another son, Karna, when she had tested the mantra out. Despite her protests, Surya the sun god was compelled by the mantra to impregnate her.