Tonight, we had a rather usual situation that somehow turned into a discussion.

When we faced the Ancient one, Y'Golonac, who had rapidly awaken from his slumber, we started fighting him using the epic battle cards.

One of our players had the "masquerade of night" card in his posessions, which could "cancel an ancient one attack for one turn".

Now, the question that did come up was:

What kind of attack can you cancel with it? Only the "The Ancient One attacks as normal" ones? Or all forms of attack, including sinister plot cards?

If the latter, are you allowed to choose if you want to use the masquerade after you saw what the attack would look like (therefore, the epic battle card being already revealed) or must you choose before you can know if his attack is modified?

I can't imagine us to be the first group to run into such an situation, but I couldn't find anything regarding it by using the forums search function...

It would be an entire turn in normal battle. So for Y'GOlonoc, if you skip the "lose 3 points of sta/san" round, your next battle would be lose 4 points of san/sta

With an Epic, you would just ignore the GOO attack on that card, still perform the Investigators portion, and then discard the card with no further effect.

So, you could also decide to block one of his sinister plot - attacks?

I would say so. As long as the card says "Ancient One Attacks" you can cancel it.

However, since the Epic Battle cards that trigger sinister plots say "Ancient One Attack" at the top, I would not allow those to be canceled, because they're just meant to be immediately replaced with an Ancient One-specific card. Not like that's a bad thing: each Epic Battle card shows Ancient One Attack on it anyway, and many allow the investigators to attack, too. Canceling the generic card that draws the specific card pretty much disallows the investigators to attack, and you won't even see what the card was going to be anyway. So why allow that?