Complex sentence: Valka encourages him by telling him that he alone can unite humans and dragons, and inspired by her words and his father's, Hiccup decides to return to Berk to stop Drago.
Split into simpler sentences: Valka encourages him by saying that he alone can unite humans and dragons. Inspired by her words and his father's, Hiccup decides to return to Berk to stop Drago.