The design on this card seems to have gone wrong... I think errata is the only way to save it.

Specifically the R. It cannot possibly work right with any of the variable lifegain cards: if I Ritual Magic off of my opponent's Battle Prowess, then (when Ritual is activated) the amount of vitality they would gain is an undefined variable, because a target has not been chosen; if I Magic off an Abelia's Friendship, the cards haven't been revealed, so again X is an undefined variable which defaults to 0.

Sorry if someone covered this already :s

Technically yes when this card is played, if there is an undefined amount, then it theoretically wont do anything. So either it can be played to just cancel a vitality gain, or you can work out the undefined amount, and then your opponent loses half of that. But youre right it is slightly poorly worded, and should be "after your opponent gains vitality" rather than "after they play an ability that gains vitality".

Is there an official response on how this is currently ruled to work?

In the cases where the vitality gain is variable, Ritual Magic would be played after the Variable is set.

Because say for example a player has Eye of the Tiger in his discard pile, (0 difficulty), Battle Prowess would generate a vitality gain of 0. Or no Vitality Gain. That would mean that Ritual Magic could not be played.

So in the instance where there is a variable ability, Ritual Magic is not played until we know whether or not there is vitality gain.

Are you sure? It seems to me that it sets a bad precedent for a card to be able to have two different triggers off the same text.

aslum said:

There isn't a trigger until it's determined that your opponent is playing an ability that will gain vitality.

That's how I'm currently ruling it.

This goes on the stack with every other ruling I've issued in the past week to talk to James and figure out what is to be done.

Let me be more explicit, this ruling would invalidate the way a ton of cards work. Canceling Friends and Rivals and Amy's Assistance and such.

I'll be honest, I've never been a fan of canceling something off of potential, but this would massively change how the game works and that I'm not so hot on.

I think its best to just look at it as a functional eratta specifically targetting this card and not having long reaching effects on other cards, at least unitll hata makes his offical on it.