If he were to march against the Empire now, as one of the most powerful Dark Lords, the Empire would undoubtedly focus its superior forces to take him down first.
Facing the onslaught of three Rank S beings, plus countless Heroes... Even if Ethan won, it would be a Pyrrhic victory.
Furthermore, this would just be doing a favor for the other Dark Lords by blocking the Empire's top-tier forces.
After such a victory, he'd still have to face a bunch of Dark Lords whose strength has surged. This deal just wouldn't be worth it.
By accepting the peace talks, he'd get a long period of recuperation while the Empire and those Dark Lords wear each other out.
Isn't it more attractive to come out at the end and pick up the pieces?
And so, on the banks of the Furious River, Ethan met with Duke Karaman, and they signed a peace agreement that would last for fifty years.
After this meeting, Duke Karaman returned to Riverside with a heavy heart.