Gyruda, Doom of Depths has made an immediate impact on eternal formats, putting a deck into the top 8 of the first Magic Online Legacy tournament where it was legal.
4 Ancient Tomb 4 Cavern of Souls 4 Chrome Mox 3 City of Traitors 4 Dack's Duplicate 1 Dread Return 4 Grim Monolith 3 Gyruda, Doom of Depths 4 Lion's Eye Diamond 2 Mox Diamond 4 Pact of Negation 4 Phantasmal Image 4 Phyrexian Metamorph 3 Progenitor Mimic 1 Sakashima the Impostor 4 Spark Double 2 Thassa's Oracle 4 Underground Sea 1 Vizier of Many Faces 4 Force of Will 1 Gyruda, Doom of Depths 2 Hurkyl's Recall 4 Plaxmanta 4 Serum Powder
With a card pool as large as Legacy's, there are a critical mass of Clone variants such that we can load our deck with enough of them to ensure that a single Gyruda trigger likely translates into milling the entirety of each player's deck.
After the combo pieces, we just need fast mana. This is really just a 1-card combo that is guaranteed to be in our opening hand, so all we need to do is resolve a single six mana spell to win the game. Lion's Eye Diamond in particular is awesome here, essentially functioning exactly the same as Black Lotus since the only card that we care about resolving is not even in our hand.
Pact of Negation is able to give us a bit of protection at no cost, since if we resolve our spell we win the game on the spot. Force of Will also works since all of the Clones are blue, but this particular list opts for them in the sideboard.
Thassa's Oracle lets us end the chain of Gyruda triggers by just winning the game outright even if your opponent had more cards in library than you. A lone copy of Dread Return is a nice touch that guarantees you can still win the game if for some reason your chain fizzled or was somehow disrupted mid-combo.