So the ATK changes to N at the start of your turn (or when played for the first time) and then returns to 0 once it has attacked? Or it returns to 0 after attacking, then gains ATK dynamically during your opponent's turn, so that you start with N attack on your turn?
Also, it wasn't stated so, is ability cost counted as well as card cost? Is the ability passive or active (lobotomizable)?
Now that the questions are out of the way, this is a really neat idea. With one it doesn't really matter much, but once you get 3-5 of these out, you really begin to rethink whether or not you need that dragon
. The effectiveness really depends on the deck you're facing. If you're playing against PDials, the effect isn't that great (no more than 3 attack per turn, I would say). Then again, if you're facing say a fractal deck, there'll be a lot of quanta spending between fractals and creatures for you to get in on.
I would be tempted to drop the unupped to 5
or maybe less, 6 seems just a tad to high. For 6
you can get a GotP with stats of 7|4, same HP but with a constant ATK of 7. I don't think the average of attack of this creature could ever compete with that. That is, your opponent won't be using an average of 14 quanta every turn, most of the time. For 5
, you can get a 6|4 toadfish; for 5
, a 5|5 abomination; for 5
, a 5|3 gargoyle. The expected stats seem output closer to these than of a 6 cost creature. Even now it seems a little tiny bit UP, or maybe it's just me. tl;dr, Upped is good, unupped is UP, because reasons.
And finally, I would change the card text to "...quanta spent
during your opponent's last turn...". Might not make a huge difference but grammar and stuff