Honestly, any deck dealing damage these days has long outgrown any fog. Between direct damage and stomp effects, can we please have one real fog back in magic?

I'm not sure if this is better or worse than Fog. On one hand, you have versatility to stop lifegain, which is strange to see in GW, but hey, sometimes design has features/flaws like that. On the other hand, this won't save your planeswalkers that are being swung at, and any creatures trying to protect them are still taking the L.

The reason there are fog breakers is that "I find a way to play this card every turn and now you cannot possibly beat me" is just not a fun state for the game to get into. Every deck should be able to find SOME answer to everything it might come up against. (Sure, you'll probably not run most of them; but you should at least have the choice to.)

Saying that - this seems like a very reasonable card to exist. It's kind of different, it has the use of countering lifegain instead which might sometimes be useful (and sensibly means it blocks both halves of a drain-life). Not keeping creatures of walkers alive makes it interestingly different from fog.

It would certainly warp an environment; burn decks and rush decks would both be seriously impacted by it; and would either be really weak in that environment, or really strong (because they get buffed up to be able to deal the extra turn of damage needed to overcome this - wrecking anyone who isn't running it). But maybe that's a deliberate choice for the environment that adds it.

against burn, is it really more broken than any life gain effect? but who plays life gain without a purpose? is sideboarding life gain common? moreover, this has timing restriction that most life gain don't.

against life gain, who plays anti-life gain anyway? for that matter, i feel this half of the effect should be black or red instead of green. Archfiend of Despair, Everlasting Torment, Erebos, God of the Dead. QED this card feels white/black.

It's potentially more potent because it's "Nope, that last burn you're casting? Won't happen. And you can't even make up for it, this turn is a bust completely." which means it scales up to as much lifegain as is needed. It won't ruin them completely, but having to build in an extra turn (or four) is going to be impactful.

I wonder if this should be "Your life total" instead. It definitely needs to disable life payments like this does though.

I think that it is good how it is; it affects everyone, including stopping life gain, life loss, life payment, etc. Note that infect does not cause loss of life and can still add poison counters, anyways. (It might be appropriate to change the color and/or mana cost; I don't know.)

White-black seems a good place for this.

Green-white could have "Players can't lose life this turn." though for clarity I might write "Players can't pay or lose life this turn."