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The situation is pretty grim for our Turbo Depths player, to be frank. The D&T player has multiple layers of disruption on board and you are under a decent clock. Many players would “go for it” and make the Marit Lage in this scenario, surmising that things probably aren’t going to get any better. If you go for it, you could use Sejiri Steppe to push damage through the Mom and Flickerwisp on the following turn. This line loses to a Karakas, Flickerwisp, or Path to Exile for sure, and another Wasteland or Swords to Plowshares would be problematic, but perhaps not unbeatable. That’s about 15 live cards for your opponent, which is not great considering that they still have cards in hand and that your gut says they have Karakas.
The big question here is actually quite subtle “Why did the D&T player attack with Mom?” Assuming the D&T player attacks with Thalia and Flickerwisp every turn, the clock doesn’t change with that one point of damage. The D&T player is obviously not afraid of anything the Turbo Depths player can do. This means that they have at least one more piece of relevant action in hand, or perhaps something like a Mirran Crusader or Sword of Fire and Ice that would prove lethal with one more point of damage pushed. Alternatively, the D&T player may be pushing damage in the face of a card like The Tabernacle at Pendrell Vale or Glacial Chasm, which would be unlikely, but not unprecedented inclusions in the deck.
After talking with Negator77 for quite some time, we believe that the optimal line begins with an Abrupt Decay on Thalia. Mana is really quite tight and you have an abundance of spells in hand. With Thalia gone, there is the possibility that you can wiggle through what is on board with the two Crop Rotations in hand. That means that the follow up play needs to be a Ghost Quarter on Wasteland to prevent losing a combo piece. This allows you to play Dark Depths without fear. The last part of this line involves using Surgical Extraction on your opponent’s draw step to fully remove Wasteland from the picture (you can tap Dark Depths to do this thanks to the Urborg, Tomb of Yawgmoth). This will also give you information about how safe it is to just make Marit Lage on the following turn, letting you know if you are going to need to save a Crop Rotation to get your own Wasteland to deal with a Karakas.
There are valid arguments for choosing a different Decay target, but mana is the biggest bottleneck here. Decaying a non-Thalia card also means that you would have to pay two life for your Surgical Extraction, which is not irrelevant at this stage of the game. That’s the difference between a 3 and 4 turn clock with what’s on board, and a 2 and 3 turn clock with something like a Mirran Crusader as a follow up play. While Decaying Mom is a valid line as well, fearing a Serra Avenger that could be both offensive and defensive, that is perhaps playing too scared of a 2-of in the deck and is a problem handled with Sejiri Steppe anyway. Reducing the mana bottleneck also allows a top decked Vampire Hexmage to be cast with a Crop Rotation open to take care of a potential Karakas (even after Port tapping a land).
That is our answer. Now, it’s important to note that the ideal line of play that you come up with after reflecting for some time and the actual lines of play you come up with in the moment aren’t always the same, even for veteran players. A number of misplays were made on both sides of the table in quick succession. Here’s what actually happened.
While he did Ghost Quarter the Wasteland and extract it, Negator77 did not end up Decaying Thalia. I believe he took out the Flickerwisp instead. My hand at the time was Cavern of Souls, Karakas, and Rishadan Port. I played the second Port and put the squeeze on his mana. A turn or two down the line, I end up Porting my own Plains due to a lag spike on MTGO. In a display of honor (and out of sheer curiosity about how the situation should play out) Negator77 responds as if I had ported his Thespian’s Stage properly. The stack gets four or five cards deep as we fight over forcing the Thespian’s Stage activation, and he accidentally makes a Marit Lage while his Wasteland is still on the stack, targeting my Karakas. Oops. We talked through the situation a bit more and realized that I still had the win regardless, but it was sort of an anticlimactic end to a really great game.