Jagon is part of the quest A Hunting We Shall Go and is located in the Temple of the Bandit's Camp in Broken Valley. You can mindread him for 400 exp for this quote: "Lord Laiken will reward me well for the head of a Slayer."
Return to Rodney in the Rivertown Barracks for a default reward of 1500 exp and 320 gold, and a choice from random uncommon items. You may choose not to turn in the quest in order to keep his necklace, which grants you +2 Strength, +1 Lockpicking, +1 Rush Attack and 2 Enhancement Slots.
He also has a random creature limb drop slot (at least in the Dragon Knight Saga version) from rotten to perfect quality, so it is a good place to save / reload when he is nearly dead for a perfect-quality creature part.
A letter written by Ragon to Carmina, titled Dirty Scrap of Paper, mentions the bandit camp and Jagon's death. It can be found in Aleroth during the events of the Divinity II: Flames of Vengeance expansion.