Why isn't the four horsemen combo a loop, like why I can't I show the potential play to the opponent and shortcut the desired result?(assuming that the desired result is no graveyard, single creature in play and a flashbacked dread return)
Let’s talk about Four Horsemen…
Four Horsemen is a really cool Legacy combo deck that is a very bad choice for tournament play because of some issues involing the rules for loops and slow play.
However, before I can get to explaining why it’s not allowed, let me tell you how it works:
The engine of the combo is Basalt Monolith and Mesmeric Orb. You can tap Basalt Monolith for mana and then use that mana to untap it. This triggers Mesmeric Orb and causes you to mill the top card of your library. Since this loop is free, it allows you to mill as many cards as you want from your library to your graveyard.
Next in the combo is Narcomoeba, which you can put onto the battlefield if you mill it from your library, which we have a way to do with the engine above. Once you have at least three Narcomoeba’s in play, you can sacrifice them to flashback Dread Return and return a target creature to the battlefield from your graveyard.
The creature you want to return to the battlefield is Sharuum the Hegemon. Sharuum allows you to return an artifact to the battlefield from your graveyard. The artifact you want to return with Sharuum is Blasting Station, which is your win condition.
So far so good, right? I mean, we just dump the whole library into the graveyard, get four Narcomoebas (who are the four horsemen from the deck’s name), sacrifice three to Dread Return Sharuum, and Sharuum brings back Blasting Station. Where’s the issue?
Well, the issue is coming, and her name is Emrakul, the Aeons Torn:
So Emrakul (or any of the three original Eldrazi Titans from ROE) is the key to turning this chain of odd interactions into an actual deck. With Emrakul in your deck, you get to reshuffle your graveyard into your library every time she ends up in your graveyard. This lets you sacrifice your Narcomoebas to Blasting Station to ping your opponent, then shuffle them back, mill them back into play, sacrifice them again, etc. And that’s how you win with the deck…
Except that Emrakul also complicates matters by making the needed loops non-deterministic. To even get the ball rolling, you need to mill at least three Narcomoebas and have Dread Return, Sharuum, and Blasting Station in your graveyard without hitting Emrakul. This isn’t guaranteed to happen, as you’re starting with a random library. It also could never happen as you could just oscillate between different failure state configurations of the deck without ever hitting a shuffle that lets you do this. (The odds of this are vanishingly small, yes, but it’s still possible.)
When you’re playing at a tournament and you want to shortcut a loop, you need to say what state the game will be in after a certain number of iterations of the loop are complete, and you just can’t do that for this combo.(You can shortcut the Monolith/Orb engine to just mill one card at a time tho, so you don’t need to keep tapping and untapping the Monolith.)
This means that you have to execute the combo manually, and that means you’ll fall afoul of the Slow Play rules, which say that “It is also slow play if a player continues to execute a loop without being able to provide an exactnumber of iterations and the expected resulting game state.”
The actual point at which the Slow Play warning will be issued is when the game returns to the same state with regards to the combo. e.g. A player controls all four Narcomoebas and has just reshuffled from Emrakul and starts milling again, looking for the combo to return Sharuum and Blasting Station, but they hit Emrakul and reshuffle again before getting there. The game has no returned to the same game state and if the player continues to execute the combo, they will receive a warning for slow play and be told to do something else.
A warning for Slow Play is upgraded to a game loss on the second and all subsequent infractions, and a player knowingly breaking the rules by playing slowly in this way in an attempt to combo off is cheating and will be disqualified.
At Regular REL, this line of play is prevented by the slow play rules in the Magic Tournament Rules, and the player should be instructed to make do something different when the game loops in the same way as described above.
For more information on the philosophy behind these rulings, here’s policy lead Toby Elliott’s blog post on the topic: http://blogs.magicjudges.org/telliott/2012/11/02/horsemyths/
