“Damn you, useless vassals! Well then, I have no choice. Witness my magic! And bow down to it's power!”
When Wagner heard Roy's group was passing through Thria on their way to the Lycian territory of Ostia with Bern's princess Guinivere, he realized they would likely stop at Castle Thria for rest. Knowing this he had Lord Orun assassinated, and he planned to invite Roy's group into the Castle saying Lord Orun was ill, then ambush the group and capture the Princess. He would then give her to Bern to earn the country's favour.
He discussed this plan with the Thrian soldiers, who were under his command, and who were loyal to him. While discussing it, Cath the thief overheard the plan. Wagner then invited Roy into the Castle, and the group was left in a room for a moment, while Wagner was gone Cath approached Roy and informed him of Lord Orun's death and Wagner's plan.
Roy then made his way to the front exit of Castle Thria with his company and was then approached again by Wagner. Wagner asked where he was going, and Roy said he was leaving unless he could see Lord Orun. Wagner then decided to simply attack and kill Roy's group with his forces, not wishing to let them go. Roy's forces knowing they were surrounded on all sides then decided to flat out capture the Castle.
Wagner is the Boss of that chapter, Chapter 6: The Trap. He is a Shaman and wears a cloak over his head that covers some of his face and gives an eerie look to him. He is the first actual Magic wielding Boss of Binding Blade, which makes him somewhat difficult. His high resistance makes him pretty much invulnerable to any magic attacks and any physical attackers, with their low resistance have trouble with his magic attacks. His throne makes it nearly impossible to hit for many units.
Wagner is killed in the battle at Castle Thria defending the Throne of the castle.
“No... My plan... was flawless...”
—Wagner's death quote in Binding Blade.