Basilisk Blood causing +0/+40 and 12 turn stasis?
How does mutation work with it?
Quintessence and rewind probably don't have anything that could be doubled... liquid shadow adds 2 poison... aflatoxin 4 poison... is that right?
Cards used on the target creature... does that include shields blocking the creature? (eg. fire buckler dealing 2 damage, permafrost freezing for 6 turns)?
All those are exactly correct.
For Mutation, I suppose it would double the bonus the creature gets to it's stats from being a Mutant, nothing more.
I suppose I should change it to "Cards affecting target creature have double their effect."
But I just realized something: How OP this card could be when combined with Anti-matter.
Look at it like this...
Crimson Dragon+Gaia's+Anti-matter=-45 attack. (Normally, it's -30, 15 to -15, but here it's -60 due to Gaia's.)
Then, if you use Anti-matter again.... It
should go up by 90 attack, putting it at 45. However, it will be doubled into +180 attack... Giving you a 135/2 creature. That's not even counting Momentum either...
In my opinion, that's way too powerful for a 4-card combo. The Trio aspect doesn't really count because you can just use one Nova to get out Gaia's, and then put out a Momentum too if you want. The only thing you've got to do from there is accumulate 11
and 11
.
So, I think I'll change the unupped version to "The next card affecting target creature has double its effect."
And then I'll change the upped one to 4 cost, leaving the text as-is.