Out of all Theros Beyond Death cards that have been previewed so far, Underworld Breach looks the most dangerous to me.
This card is reminiscent of combo powerhouse Yawgmoth’s Will. It’s potentially even better because it costs only 2 mana, and because you can keep recasting the same spell as often as you like. This does require you to have enough cards in your graveyard to keep paying the escape cost, but there are milling engines that enable this.
Underworld Breach in Modern
If you start the turn with Grinding Station and a zero-mana artifact on the battlefield, then Underworld Breach allows you to mill your entire deck: You sacrifice the artifact to Grinding Station, recast it by exiling the cards you milled, and loop. Eventually, you cast a lethal Grapeshot from your graveyard for the win.
Casting Grapeshot does require 2 mana. Controlling two untapped lands might work, but the real plan is to control three artifacts and tap Mox Opal for mana in every iteration. This additional mana will also allow you to cast Grapeshot a second time if the first was not lethal yet (which could easily happen because a 60-card deck can generally escape only 16 spells). Here is a rough list, filled with cheap artifacts and mill effects.
Grinding Breach Combo
4 Scalding Tarn 4 Misty Rainforest 2 Darksteel Citadel 1 Prismatic Vista 1 Inventors' Fair 3 Snow-Covered Island 1 Snow-Covered Mountain 1 Snow-Covered Forest 1 Breeding Pool 1 Steam Vents 1 Stomping Ground 4 Emry, Lurker of the Loch 4 Mishra's Bauble 4 Mox Opal 4 Arcum's Astrolabe 4 Shriekhorn 4 Grinding Station 4 Underworld Breach 4 Thought Scour 3 Noxious Revival 3 Muddle the Mixture 1 Engineered Explosives 1 Grapeshot Sideboard 3 Veil of Summer 3 Mystical Dispute 3 Nature's Claim 2 Pyroclasm 2 Engineered Explosives 2 Oko, Thief of Crowns
Noxious Revival can bring back Underworld Breach after we mill it with, say, Emry, Thought Scour, or Shriekhorn. As a result, we will be able to find the enchantment consistently, and we should be able to go off by turn 4 in most games. With the perfect opening hand, we could even kill on turn 1:
This deck could be pretty busted.
Underworld Breach in Legacy or Vintage
In Legacy or Vintage, alternative combo chains arise. For example, you can cast Underworld Breach, Brain Freeze yourself, and exile those cards to cast Lion’s Eye Diamond from your graveyard. The mana from Lion’s Eye Diamond allows you to cast Brain Freeze; the milling from Brain Freeze fuels escape; and you can eventually point multiple Brain Freezes at your opponent for the win.
Alternatively, you could cast the same Manamorphose over and over while keeping a dredge chain going with Golgari Grave-Troll and Stinkweed Imp. This does require you to dredge into a new dredger every time, but if you find Lion’s Eye Diamond along the way, then all will be well. All of these combo engines seem pretty powerful.
Underworld Breach in Pioneer
In Pioneer, you could set up combo loops via multiple Diligent Excavators and Mox Ambers, but it seems easier to use Chronic Flooding and Hidden Strings. Hidden Strings untaps lands to pay for itself, and the trigger on Chronic Flooding helps pay the escape cost on Hidden Strings. After milling your entire deck, you could win via Jace, Wielder of Mysteries. Whether this is better or worse than Lotus Storm remains to be seen, but it’s an interesting option.
Underworld Breach in Standard
In Standard, it is more difficult to set up free mill engines. Looping two Emry, Lurker of the Loch via the legend rule could work if you can generate a mana every loop, and it is possible to accomplish that by combining Jiang Yanggu, Wildcrafter + Grumgully, the Generous + Samut, Tyrant Smasher. However, this seems very convoluted and won’t make for a competitive combo deck.
In older formats, powerful broken combo engines arise with the right support cards. I expect Underworld Breach will see a lot of play.