Since you are fire the cursed will basically just help amplify the damage of your demon and merc, breaking any physical immunitie.
While I think less important still useful, conviction aura should be your main concern while farming a new Hephasto.
Great, thanks! that's what I was thinking as well. So cursed would be an upgrade so that the kill speed of my merc /demon increases, which is especially helpful for immunes.
I have also thought about maxing blood oath to have 43% of dmg taken going to my demon (who tanked every attack of dclone). Any experience with blood oath?