Complex sentence: The Kid learns that the Journal belonged to Zia's father, Venn, who had worked for the Caelondians in building a weapon intended to destroy the Ura completely to prevent another war.
Split into simpler sentences: The Kid learns that the Journal belonged to Zia's father, Venn, who had worked for the Caelondians. He had helped build a weapon intended to destroy the Ura completely to prevent another war.