Ok, thanks for all the comments, I'm still not entirely sure where I want this to go, but this is a good start.
Thematically, what is stopping the Boa Constrictor from paralyzing the opponent and only allowing him one card?
I'm going to go ahead and ask what happens if I have six of these on the field and play shard of patience.
The amount of restricted cards is based on the Boa Constrictor with the highest attack, so it doesn't matter how many there are (although I could change that, the mechanic could be 7 minus the amount of Constrictors on the field).
Or, it could be that every turn, the value of X is lowered by one until it reaches one and then stays there until the Boa that has that count is removed from play, if that makes any sense.
Turn One Two Three Four Five Six Seven Eight
# of Boas Send One Send Two Two Two One (kill) One
Card Use Six Five Four Three Two One Two One
Send: you cast a Boa Constrictor | Elite Boa
Kill: First Boa is killed (or removed from play from cards like RT)
If that table makes it any clearer (which it probably doesn't, but let's pretend here), that's how I think the mechanic could possibly work.
Also, I've decided that Constriction (I like the name, thanks Frozengaia
![Cheesy :D](https://elementscommunity.org/forum/Smileys/solosmileys/cheesy.gif)
) is a passive ability, although I suppose having it as an active ability would allow it to be countered by more than just hard CC.
And as for thematics, I imagined the Boa Constrictor slowly squeezing the opponent and gradually stopping movement and action, not just straight up paralyzing and immediate effects.