The new effect copies a target card (this implies targeting the card in the hand) and puts it randomly on top of your or your opponent's deck.
Imagine a generic Fractal Deck. Fractal, Pillars of Element X, Cardinals, Fractal Fodder from Element X and an Aether Mark.
The Aether mark will make your Cardinals Immaterial. This is important since they play a major role in the new deck strategy.
Most of your opponents would not have a use for multiples of any single type of card from your deck.
Each use of Cardinality will have a 50% chance of determining your next draw OR depriving your opponent of their next draw.
With enough quanta and enough Cardinals you can ensure your opponent never draws another useful card unless they are the same Duo as you or have a CC shield.
How expensive does Cardinal/Cardinality need to be to balance this effect?
Imagine a deck that would greatly appreciate a 7th copy of a permanent (Dimensional Shield, Sundial) or spell (Fire Bolt, Miracle, Supernova).
Each use of Cardinality has a 50% chance of giving you that 7th+ copy at the cost of losing your draw.
How expensive does Cardinal/Cardinality need to be to balance this effect?
