It's intended to be used in a stall.
Right...I admit I made a logical and contradictory mistake with my first sentence. (I edited my post)
However, the rest of my point still stands.
Why does it have to be immaterial?
As I see it making it immaterial both solves but creates a problem.
Problem solved: If it is not immaterial, then having zero cards left, if the opponent PCs it, you instantly lose the game despite your current HP ( I assume this is the case)
to prevent this, it must be immaterial. (since there can only be one on field)
Problem created: If it is immaterial, you have zero cards left yet if you're out-healing the maximum damage output of your opponent, then essentially its an auto-win if your opponent lacks PC for SoGs or empathic bond, staff etc. and/or mass CC if you're using empathic bond with mitosis + cheap creature. The opponent ultimately ends up decking out. (which is the desired effect obviously but the potential lack of counter play is somewhat problematic)
On top of that, if your opponent is also using a stall, yet lacks this card in their deck, you have effectively out stalled your opponent, almost effortlessly, causing them to deck out first no matter what they do.
However, as my previous post implies, One cannot assume that you are always going to set this up perfectly and survive even before you reach zero cards and having this card on field.
That said, the instant immateriality of the card and its condition of having one only on field creates another problem which I have already stated.
Problem created 2:
If you have one of these on field, then any other within your deck be it 1 or 5 more (6 max) will effectively delay your deck by wasting card space.
So one can assume that you can't really have more than 2, MAX 3 to minimize the inevitable delaying effect it will have.
Solution:
To avoid both problems it creates, I propose removing immateriality AND the condition of having one only allowed.
why?
First, Immateriality is already very possible with PA.
That said, the need of PA makes the setup only slightly more difficult while giving at least some counter play for the kinds of scenarios mentioned previously.
Also, having PA can easily benefit a
deck using fountain, if you don't have fountain, then start PAing Staff or SoG (or empathic bond or something)
Secondly/ alternatively, IF you don't want to use PA, then all you have to do is be able to have multiple on field.
The cost of 7
in this case makes this somewhat balanced as I mentioned that besides the anti-deck out, it is also a more expensive SoG, which is fine in this case.
Since immateriality is removed, it can also be somewhat slightly cheaper (upgraded?) so that it does not have to be locked in
and can offer its effects to other elements, thereby satisfying a broader player base within the game.
As it stands right now, I feel as though the idea is good but is somewhat problematic.
but, its up to you. just my 5 cents.