Correct it was his second in command Victor from his dragon slaying corps days who backstabbed & betrayed Leon during their first assault of the silver dragon clan. The Easter egg from this is the small piece of Leon's armor (Black Gold War Chariot) that Noah has in her box. It's from the hole that was made when Victor stabbed Leon from behind and left him for dead. The King & Queen of the empire feared that Leon had grown too strong & powerful to control and thus interfere with their grand plans, so they chose to get rid of him from the shadows and asked Victor to do the deed.
What's the Grand plan?, In Leon and Konstantine's second meeting he revealed 30 years ago in the timeline when the war was at it's peak the empire stuck a deal with the dragon kings to prolong the war. This way that they could reap maximum profits from the population without raising taxes or changing the labor laws. The longer the war drags on, the more manpower, finances, and resources the people funnel into supporting it. In conclusion this is all about ripping the population off under the guise of manufactured war.