a good conversion would be to put an executioner on a warp spider exarch by cutting off the death spinners and the powerblades and gluing on the executioner. This would make the model worse to use butwould look great. Maybe to keep the exarch with a ranged weapon you could put Maugan Ra's Maugetar(shuriken cannon with built-in executioner) on. Maybe you could use this as a 7th Phoenix Lord