The verdict is in, and the most powerful card in Ikoria is undoubtedly Lurrus of the Dream-Den.
I'm not going to go into formats other than Standard here, but rest assured it doesn't matter what format we are talking about, Lurrus is making a big impact. In Standard it is the sacrifice decks that have really been tipped over the edge. Remember that before Ikoria, sacrifice decks were already tier 1, so this is pretty scary. Still, there are a few different directions to go with your Witch's Oven/Cauldron Familiar deck.
I want to be clear though, it is always going to be right to have Lurrus of the Dream-Den as your companion in these decks, which means you can't maindeck any copies. Luckily, having your nonland permanents cost one or two mana is very manageable.
This is the most popular take on Lurrus of the Dream-Den right now, and probably overall the most popular deck in Standard. It looks very similar to what Rakdos Sacrifice was last season.
This is the version I decided to play on the ELEAGUE Showdown, so rest assured I really do like the deck:
The deck does not get to play Mayhem Devil anymore, but the sacrifice is definitely worth it, as you always have Lurrus of the Dream-Den you can cast on turn three if you want to. Usually though, it is best to wait until later to cast Lurrus, so you can immediately play something out of the graveyard the turn you cast it. Since your permanents are so cheap you want something to be able to sink your mana into once the game goes past turn three. Castle Locthwain is of course good for that, but so is Whisper Squad, as being able to search out additional copies is really nice.
When looking at the creature base, initially it looks a bit underpowered. Whisper Squad and Serrated Scorpion aren't creatures that you can get excited by just by looking at them. It's only once you get a chance to play with them that you realize their importance in the context of the entire deck. This list often drains the opponent out for those last few points, and while Cauldron Familiar plus Witch's Oven definitely helps with that, so does Serrated Scorpion.
We see some familiar hits like Kroxa, Titan of Death's Hunger as another play you are looking to set up in the late game. However, there are also cards like Rix Maadi Reveler that were not usually seen in previous versions of Rakdos Sacrifice, and are going to be in this version. Lots of one and two-drops opens the door for Call of the Death-Dweller as a way to bring those creatures back from the graveyard. The deck has a ton of power, but perhaps the biggest incentive to playing red is actually Claim the Firstborn as a way to really disrupt opposing creatures, and then sacrifice them to your benefit.
This version of Rakdos is pretty close to normal at this point, yet there are many other directions you can go with a Lurrus of the Dream-Den-based deck. Rather than play a second color, it is reasonable to go straight mono-black. You don't lose the core of the deck, and the mana becomes much more consistent.
Here is a list from Ondrej Strasky.
This deck is fairly streamlined when you think about the card pool you have access to once imposing the Lurrus of the Dream-Den restrictions and then only playing black. The idea is to play all the best black cards, but there are a few decisions that are important. We see Gutterbones and Knight of the Ebon Legion in the one-drop slot, and no Whisper Squad. However, I have also seen versions of this deck that do have Whisper Squad as well. If you choose to play Whisper Squad it is to have a more grindy gam eplan, rather than pure aggression.
Fiend Artisan is another card I have seen come in and out of the deck based on which version you are looking at. The good thing about Fiend Artisan here is that it is easy to cast on turn two, because the deck is only one color. However, we don't see a dedicated toolbox of creatures to tutor up, or as many creatures that are great sacrifice fodder, compared to some other variants that play Fiend Artisan. This leads to it being a three-of. Priest of Forgotten Gods will always be the best two-drop, but after that, Mire Triton and Fiend Artisan share that secondary role.
As far as removal, Heartless Act is the new Doom Blade, and so we see it here as the deck cannot play Murderous Rider due to Lurrus of the Dream-Den restrictions. Heartless Act will kill what you want most of the time, but there are some circumstances where a creature gets counters on it, which is pretty annoying. Tymaret, Chosen from Death is a great way to disrupt the opponent's graveyard, which is quite important in these Lurrus mirrors. The deck loses a bit of power but is more consistent because of the mana, which is very typical of a monocolor deck.
Another direction to take Lurrus of the Dream-Den is to actually have white in the deck. Orzhov allows you to gain access to some additional sacrifice synergies. The deck actually took down a large event run by Lotus Box this past weekend, and seems to have a bit of an edge in the sacrifice mirror matches. Here is the list by Robert Stanley:
One noticeable inclusion is the full playset of Fiend Artisan. The card becomes more difficult to reliably cast as Plains start to be put into the deck, but on the other hand you can see why Robert wants to run the card. White offers access to Hunted Witness, which is really the perfect creature to sacrifice to Fiend Artisan. There are also now a wider a variety of two-mana creatures to go find if you do sacrifice one of the one-drops. Lampad of Death's Vigil is a silver bullet that allows you to immediately have a creature-based sacrifice outlet that can also kill the opponent.
Perhaps the biggest incentive to play the white is Cruel Celebrant. This card makes the life draining that much more effective, and things really start to spiral out of control once the Cruel Celebrant gets a bunch of triggers off. We don't see Serrated Scorpion here, as something needs to get cut as cards get added, and you would rather have Hunted Witness I think.
The other nice part about playing white is having a better sideboard. The Lurrus of the Dream-Den decks are well known now. Players are going to throw the kitchen sink at you, so beating sideboard hate becomes very important. Hushbringer shuts down some decks in the format, like Gyruda, Doom of Depths combo. Even though it hurts your Cauldron Familiars, this is the type of high-impact card I'm talking about, as it can win the game by itself in the right matchup. Despark is also perhaps the best removal spell against Jeskai Fires, as it deals with all the permanents you truly care about, and the same can be said against Temur Reclamation. The list goes on, and the sideboard provides more flexibility with access to that second color.
While there are many directions to take Lurrus of the Dream-Den and I can't talk about them all thoroughly in this article, I would also like to touch on a Mardu version. This isn't as proven as the other three decks I mentioned, but there is a lot you can do once you choose to play three colors.
This list is a bit wonky, but provides a picture of yet another direction the strategy can go in. When looking at the removal suite, notice the four copies of Dire Tactics. As it happens many of the creatures you already want to play are Humans, so having this as a removal spell makes total sense. Corpse Knight serves as copies five through eight of Cruel Celebrant for even more life-loss triggers.
By playing three colors you no longer have to be worried about running sub-par cards. It actually becomes about picking and choosing which cards are the most important, as there are a ton of options to choose from. I like having plenty of answers in the sideboard to Grafdigger's Cage, one of the more annoying hate cards seeing play right now, and Embereth Shieldbreaker can be a nice answer to it.
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Lurrus of the Dream-Den decks are going to be a major player in Standard for as long as the card doesn't get banned. We are starting to see players adapt by having cards like Cry of the Carnarium and Grafdigger's Cage as high-impact sideboard cards, but even so Lurrus continues to perform, and is the strategy to beat right now. What little graveyard disruption is legal in Standard just became much more important, but these decks can simply win by attacking even if the sacrifice plan isn't working properly. It should be interesting to watch how Lurrus-based decks evolve over the next few weeks.