This question already shows the fate of most new cards with a very promising power level. The first Modern Horizons, for one, was filled to the brim with them. While Urza and Yawgmoth, after finally being printed as creatures, gave birth to entire new archetypes focused around themselves, some cards rather added to existing strategies. Take Seasoned Pyromancer, for instance. The Human Shaman still remains so powerful that it's truly a question whether or not its full potential has been reached. Not only was it responsible for the resurgence of existing decks, such as Gruul Midrange, it also helped shape some (relatively) new decks. One such deck is (the relatively new) Monored Aggro, which took the Monored Prowess shell and mades some improvements. I expect Grief to take a similar path.
In short, Grief is almost a strictly better Unmask, although that "almost" is probably redundant in Modern's case. On one hand, it is a creature, so it avoids Force of Negation and all other noncreature counterspells. On the other, you cannot target yourself with it, but again, I don't think this is of much concern in Modern. Also, being a creature and having an enter-the-battlefield trigger greatly raises this card's power level, making for some crazy combos.
It's obvious it'll work great with a certain combination of cards, but first, let's see if it'll work by itself.
I think it is pretty obvious too that Grief won't even need a combo shell, whether we like it or not. Above all, this card is a serious combo deck killer itself, as it can disrupt a deck's strategy at not that serious of a cost. Coupled with another discard spell like Thoughtseize or Inquisition of Kozilek, it's serious disruption however you turn it, especially on turn one. While combo decks indeed can usually brush off one discard spell, how well can they deal with two?
If you're on the play, you could always, for instance, take both Amulet and Arboreal Grazer from Amulet Titan. Taking both Expedition Map and Sylvan Scrying from Tron is also an option. The possibilities are endless, as such an opening exceeds utility against pure combo decks. This can hurt basically every strategy in the format, therefore increasing the chances of Grief becoming a black main-deck staple. In that case, deck builders will definitely come up with ways to support it.
That was from a disruption standpoint, but what about protection? You can also use Grief to protect your own game plan. This gets much better if you're playing a black combo deck such as Ad Nauseam. While it isn't really at the forefront of competitive play right now, it's still holding up, and now that might change for the better. Or perhaps this is exactly what a blue-black tempo build needs? Turn one Delver dies to mostly anything, so this might increase the chances of its survival.
Another big thing to keep in mind is how Grief can pitch "useless" cards in game one, increasing its effectiveness. Black decks in particular are often filled with main-deck removal like Fatal Push, which tends to be useless every now and then. If you know what deck your opponent is playing, for instance in casual play, or if the player is well known for a single archetype, especially online, that may also give you a slight edge. There are so many small details about this card, it's hard to keep track of all of them. All of them make me think that Grief is a great card with not that high of a building cost. It does require you to play black as a primary or at least secondary color, but black already does have such a role in Modern anyway.
If you haven't heard it already, Grief plus Ephemerate is one of the most anticipated turn-one plays in future Modern. How wouldn't it, really? By your second upkeep, you'll have a 3/2 creature and your opponent is about to discard a third nonland card.
This is so troublesome at first sight that I can't see why it won't be at second. All you need is Grief, a black card to pitch to it, and a blink spell. Now, of course that's kind of asking for a lot on turn one. But I think that this play will remain staggering even in the later turns, if a bit less, depending on the matchup. The biggest issues here are that all the best Modern-legal flicker spells are white and that we need additional worthwhile targets for them, few of which are black, while keeping enough of a black component to pay Grief's evoke cost.
Another path to consider is using pseudo-blink effects like Undying Evil. Black offers a wide range of them, with Malakir Rebirth at the forefront. Its interaction with Grief is of a far lower power level than Ephemerate's as you miss out on the rebound. However, the costs in deck design are even lower as you could stay monoblack. Not least, Malakir Rebirth almost acts as a modal three-sided card here: the front combos with Grief, the back is a land if need be, and the third mode is paying for Grief's evoke cost.
Finally, sacrificing Grief with the evoke trigger on the stack is another way to avoid card disadvantage. Village Rites for example turns what would otherwise equal a two-for-one trade against you into card parity. (You invest three cards to get two from your library and the best one out of your opponent's hand.)
There is only one deck in the current metagame that utilizes both white and black: Orzhov Stoneblade. Its main strength, especially for our purposes, is it already uses Ephemerate because of creatures with great enter-the-battlefield triggers. Here's what I think a new list might look like:
Like I usually tend to do, I took an existing list that performed well on Magic Online and thoroughly changed it to showcase new cards. Sword of Hearth and Home might also work well here since it can also flicker a creature, although I am aware that this might be going overboard.
I've created this list with the goal of making each and every creature an Ephemerate target, which probably shows. Tidehollow Sculler and Skyclave Apparition work particularly well when ephemerated with their enter-the-battlefield on the stack (although the mana is admittedly hard to pull off). If they leave the battlefield before they've exiled a card, the corresponding ability won't ever give your opponent anything back. Then they re-enter the battlefield, exile one card, and finally exile another when the first ability resolves. You essentially double the upside for half the drawback.
I also added a Void Mirror to the sideboard here. It hits some of the same matchups as Damping Sphere, while extending its reach to stuff like Force of Negation. While the card might severely hamper Eldrazi Tron, Green Tron, and/or Amulet Titan, it remains an uncertain prospect, partly because it also counters a free Grief, though that becomes less relevant in the later turns anyway.
We're living in an era where this weird Rakdos Waste Not discard deck already is a thing, so why not experiment with it? Notably, Grief allows us to reap some benefit right away after casting the enchantment rather than wait a turn. Some builds rely on Lurrus, while others dump the kitty Cat for Sedgemoor Witch and Young Pyromancer. I took some inspiration from both.
This list is further proof of how black cards can easily be exiled for Grief if drawn at awkward times. For example, I love how carelessly this deck can include Leyline of the Void. If you don't draw it in your opening hand, but get to it early in the game, you can exile it for Grief's evoke cost. The same thing goes for, say, Dream Salvage and Sedgemoor Witch, which is why I've gone for a three-three split between the latter and Young Pyromancer. Otherwise, I probably would have cut another Witch.
Grief isn't the only creature that's making Malakir Rebirth great here. Kroxa is another option, as it'll also make the opponent discard an additional card. It will still be sacrificed upon entering the battlefield, unlike Grief, but at least it can make your opponent discard a land and lose 3 life in the process.
If you're wondering why, of all decks, I've decided to cover Grief in a spell-based deck with Sedgemoor Witch, it's because it was heavy on discard anyway. Here we have a fragile class-cannon strategy that requires heavy disruption backup to work, and Grief helps with just that. If I were to cover nearly any other deck, I'd just cram a playset of Grief into it without changing the list any further. I don't think that would make for much interesting insight, though I believe that to be a good idea for many black decks in the future of Modern.
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