Cling to Dust
This card looks benign and doesn't happen often, but is effective if it comes up. Even though Yorion leaves us with a less consistent 80 card deck, the risk of getting two of these in hand is not worth trying to increase consistency since it does nothing on its own. What it does is over the course of the game it stops reanimation and escape creatures while also periodically drawing you a couple of extra cards or gaining life as a mana sink in your graveyard. This deck has so much card draw and so many spells that it will not be difficult to fuel the escape cost if you have free mana.
Omen of the Sea
This is a very good card; it grants you selection to filter through this large deck, either by digging for needed lands, or finding specific hate pieces that work well in your matchup. This also gives you something to do if your opponent doesn't play anything for you to answer, and having one or two of these out allows Yorion to gain extra value when summoned. This also gives you a scry 2 in the very late game, which is very helpful when your opponent is top decking and you need to keep drawing gas and answers.
Narset, Parter of Veils
This card shuts down Uro, Titan of Nature's Wrath and the cycling deck while also having incidental value from stopping card draw. She also lets you dig through your deck for the spells that you need, giving good card selection and advantage if not addressed. She can also be flickered by Yorion for additional value.
Teferi, Time Raveler
Although Teferi is very good and borderline broken in most decks, here it's a mediocre card. He offers some very unique utility by bouncing opponent's stuff when it is inconvenient, buying back an Omen of the Sea or Murderous Rider for value, and in some cases winning counter wars or turning Extinction Event into an instant-speed board wipe. However, he cannot be abused, and since our deck is not trying to push through a singular threat, he is not the most important attrition piece in this deck.
Chemister's Insight
An instant-speed spell that draws you cards is good, play it whenever you need to refill your hand and your opponent doesn't have anything you would like to answer. Be aware of this card when playing, so if you are getting mana flooded don't be afraid to sandbag a land. That said, this deck typically has plenty of cards and ways to use excess mana, and you'll probably draw a land to pitch to this anyways within a couple of turns, so don't worry too much about it. This also lets you trade in more situational cards that are no longer useful. Also, be careful and intentional if you happen to exile this to escape Cling to Dust.
God-Eternal Kefnet
This is another mediocre card; since we run a low creature count, most removal will be pointed at Kefnet. However, Kefnet offers a good body and threatens to beat down if we have control over the board. If you get even one reveal trigger from Kefnet, you will also gain some good value. Keep in mind that flashing Omen of the Sea on an opponent's turn creates a "first card drawn" trigger that you can use from Kefnet if you have mana available, which extends to counterspells.
Enter the God-Eternals
This is a very good value card, but is singleton since some matchups this does not have good targets. Removing an opposing creature, gaining some life, and presenting a threat gives you a frequent 3- or 4-for 1; there isn't much to say about this. You should also target yourself with the mill, as that would fuel Cling to Dust and could land a jump start for Chemister's Insight.
Ashiok, Nightmare Muse
This will usually win you the game if your opponent does not have an immediate answer. A set of 2/3 bodies is very potent, and Ashiok's minus ability also punishes opponents for topdecking or does a nice Teferi impression. If you get the chance to ultimate, you can beat your opponent with their own strategy; especially since this deck has a fair amount of incidental exiling.
Shark Typhoon
Oftentimes this will give you a surprise blocker and beater against more aggressive decks and if you're trying to wrap up the game against a degenerate deck before they combo off. This also is very good to play when you stabilize, and if left unanswered will grant you with an army of beefy flying sharks.
