Ring of Vile Intent has a normal and an empowered version, however no mythical. The only endgame ring with flat Poison Damage over time is Ring of the Black Matriarch but it comes with attack speed instead of casting speed. There is no MI ring with Poison Damage over time either.
Yes, there are many epics to choose from already. But this I feel is an item that could close a niche and should be considered to be added.