Join Forces was hinted at in the achievement card for the Commander release parties (I'll be attending one in Calgary... check out your local scene to see if there's one near you) and this is the prototype for the new mechanic which I will be shaking tings up.
(I considered doing a slow reveal on this one or forcing people to collaborate to assemble the pieces of the card image since that's what this card is about... but I figured you guys have waited long enough
Alliance of Arms is tricky... most multiplayer strategies are. At first, I wasn't sure how good it will be but then I realized there are two different ways to break the card's symmetry. Doing so with other cards, like Overrun, Rhys the Redeemed, or Concerted Effort, is fairly straight forward.
But things get far more interesting when you consider it in light of other *players*. In the simplest incarnation, an ally (temporary or permanent) can attack a shared enemy, forcing them to block and trade. That leaves the caster with a horde to beat down with. Of course, the ally may not want to commit all their forces... so things get interesting.
In fact, there are tought choices to be made with the card even before we get to attacking. As an opponent, do I want to spend mana on the spell at all? Presumably the caster has ways of making it asymmetric... but I get to attack with them first. Or, in light of other players, do I? Also, because the "additional" mana which constitutes X isn't spent until the spell resolves, it's quite a nice threat in a counter-heavy environment. It's a one-mana spell which could be game breaking.
Alliance of Arms, and other Join Forces cards, aren't straight forward but then that's the beauty of good multiplayer games... the complexity. For those that can master them, these cards may prove to be Commander powerhouses.