The combo itself is fairly simple. You need a Legendary creature, Grinding Station, Underworld Breach, and a 0-Mana Artifact (ideally Mox Amber). The combo works as follows:
With a Legendary creature in play, tap your Mox Amber for mana.
Activate your Grinding Station, sacrificing your Mox Amber and targeting yourself. Mill three cards.
Cast Underworld Breach. Use the three cards you've milled for the escape cost to re-cast your Mox Amber from your graveyard.
When Mox Amber enters the battlefield, Grinding Station will trigger. With the trigger on the stack, tap your Mox Amber for mana again, then sacrifice it to the Grinding Station.
Rinse and repeat this process until you have 0 cards in your library. At some point in this process, you will mill over your Thassa's Oracle. When your library is empty, cast Thassa's Oracle from your graveyard with Underworld Breach.
Win the game.
This deck attacks your opponent from multiple disparate angles. Did your opponent bring in Force of Vigor and Boseiju, Who Endures? Great — you're attacking them with a Ragavan, and a 3/3 Dragon's Rage Channeler while your Emry draws you an extra card every turn with Mishra's Bauble. Did they overload on creature removal to kill your turn 1 one-drop? Well you've followed up that one-drop with an Urza's Saga and started making constructs. Have they tapped 2 mana on turn 4 to Dress Down away your two 5/5 Construct tokens? Sounds like it's time to go to your second main phase and drop Underworld Breach with Spell Pierce backup, therefore comboing them out of the game.
Sideboard:
Defense Grid - Great against control. As an artifact can be played multiple times with Emry.
Tormod's Crypt is better than Soul-Guide Lantern because it's 0 mana. It helps accelerate Emry, and against the decks you want graveyard hate for, you'd rarely be drawing a card with the Lantern anyway. You can also combo with Crypt as a 0 mana artifact in a pinch.
Engineered Explosives is lights out against Hammertime, and very good against Death's Shadow decks. It's also the only way for this deck to permanently answer Rest in Peace, albeit inefficiently. I play 3 because in the matchups where looping EE with Emry is game over, I want to see it every time. I don't always board in all 3 though; it's fine to bring in 1 or 2 to hedge against your opponent's enchantment-based sideboard hate.
Alpine Moon - Previous Breach players have played Spreading Seas, which I think is terrible in this deck. You want something efficient more than you want something that draws a card. I don't know if you care enough about Urza's Saga decks that these are necessary, but they've been good for me so far!
Maybeboard:
Pyrite Spellbomb - I'm not sold on this card, but it's good with Emry and kills Sanctifier En-Vec, which may or may not matter. I could see replacing this with something else until Sanctifier starts seeing more play again.
Aether Gust to beat the green hate spells and Titan
