Complex sentence: Afterward, the Underworld Commander gravely wounds Ilias, who before dying cuts off one of Wyvern's horns, as he promised, leaving his son Regulus, believing in his future and his bright star.
Split into simpler sentences: Afterward, the Underworld Commander gravely wounds Ilias, who before dying cuts off one of Wyvern's horns, as he promised. Leaving his son Regulus, believing in his future and his bright star.