Started with the name and went from there. Not sure about the mana cost, since the effect's pretty strong. Thoughts, criticism welcome.

The closest I can think of is Furnace of Rath, which is symmetrical but hits creatures as well. This may actually not be as strong as it looks, since if you're getting damage through to your opponent you're often winning already.

Or, in theory, this could be a spell that deals 7 damage to the opponent. Personally, I think Sorin's Vengeance is undercosted, but one can't ignore the fact that it exists. If there is no life gain/damage prevention, then dealing 10 damage to a player will always be superior to dealing double damage (because you didn't have to set up the enchantment before dealing any other damage.)

That said, I think the ability is good, and should exist. It just needs to cost less. I also wonder if the wording "Whenever an opponent would take damage, they instead lose twice that much life" would be better? I'm pretty sure your version isn't recursive movezig, but it still kind of bugs my head. Maybe I'm just buggy, though.

You're just being buggy, jmg. See for example Furnace of Rath. Replacement effects never replace themselves, and as you can see, this template makes enough sense to see print.

Well, this has the confused "Whenever...instead" wording that is never seen on real cards. "Whenever" means a triggered ability, but "instead" means a replacement ability.

I assume movezig5 wanted this to say "If an opponent would lose life, that player loses twice that much life instead", which is indeed a printable template, something like Gratuitous Violence meets Wound Reflection.