Enma is not only the king of hell, in mythology, but also, and more importantly, the judge of all hellish inhabitants.
If Zoro has a serious battle with one or more Gorosei, Enma thematically is bound to provide the finishing blow. If Oda sticks to the meaning behind the sword's name, then the Gorosei's hax regen/immortality must have a counter in haki, and nowhere else.