Neat. I can't come up with anything better than a Combustible Gearhulk (and its cycle) or Sundering Titan to abuse it. Except that's already quite an abuse...

I feel like you hit enter by accident.

Ah, you edited while I typed.

Those are both pretty good options, though.

Turn one Concordant Crossroads, turn two this into Blightsteel Colossus

Surprisingly, there isn't an artifact creature with haste that's big enough to warrant a warning. Lunar Avenger has sunburst. Most of the other six cost cards require some sort of set up. Snare Thopter may be the most dangerous (?) of the lot, which is really telling.

The quickest way to figure out if this card is broken or not is to go pick up a Trash for Treasure deck, since this seems to be its territory. But a number of good Welder tricks go away when artifacts don't die. Cards like Solemn Simulacrum and Spine of Ish Sah are only half as good as they could be.

­Possessed Portal may be the strongest simple trick I can think of. If your deck is a creature rush deck that can operate on only two mana, this combo can put a real squeeze on your opponent. Duplicant, as per usual, is a good. But again, it's a two for one. Triskelion is a little less 'feel bad', but more often than not it will be just as useful as a Tremor.

­Magister Sphinx may be one of the most broken simple combination, though. Red can only dream of dealing 10 damage on round 2 with two cards.

Sure, if you want this as part of a 2 card combo then you're 'only' going to be able to sundering titan or gearhulk.

But as dude said, any haste enabler and this is busted as all heck.

Well... Ish. Not as "busted" as Master Transmuter or Through the Breach. Basically certainly no worse than anything Wizards have printed themselves.

The power of these effects comes with lower CMC though. The earlier you can drop your effect, the sillier.

By turn 5 a Constructed deck could have an answer or already present you with pressure of their own. What chance is there of the same turn 2?

I still think the Magister Sphinx combination is troubling. But the worst that would happen is that Wizards bans Magister and Titan in Modern, and that might be a reasonable way to go forward.

But it is kind of strange that Proof is reasonable only because of the card type. Had this said 'Creature' instead of 'Artifact', it would be about as broken as Flash (if not more so.) And I do think a reasonable counter-argument could be "This might not be broken now, but it's going to potentially prevent us from printing a lot of cards that it could be broken with."

I'm not sure if I agree with that argument. I just think it's a valid viewpoint.

So, uh; as broken as a card they actually designed, developed, playtested, and printed? That seems like a perfectly reasonable bar to aim for.

@Vitenka: As broken as a card that is either banned or restricted in every format you can play it in? That argument would deem Black Lotus as 'fine' since it was printed.