[Update #2]: The day has finally come - MTG Arena has now made its way onto Steam!
[Update]: MTG Arena's Aftermath mini-expansion is coming on May 11th with 50 new Standard-legal cards.
After a fairly lengthy round of teasers, MTG Arena's massive March of the Machine expansion has now finally been unleashed. It brings with it a whole host of new cards to mess around with, a bonus batch of popular legendary creatures from previous sets, and most importantly of all, an entirely new card type to shake up the gameplay loop - Battle.
Your Battles enter play on the opponent's side of the board with a certain number of counters on it, sort of like a Planeswalker. Once you bring those counters to zero through either damage or effects that remove counters, you'll be able to flip your Battle to its backside and unlock a fairly powerful reward. Notably, "destroying" Battles directly will not flip them - you need to remove all of the counters 'fairly' to get to the good stuff!
It's a rather simple concept, so if you're even remotely confused, don't worry as you'll figure it out within moments of playing your first Battle. For now, allow me to give you an example as that'll likely answer some of the questions you might have. Have a peek:
As for the previously mentioned legendary creatures, also known as Multiverse Legends cards, those will be added to each pack as a little bonus, and this includes draft packs as well. Best of all, there will be duplicate protection for these cards! So if you already have 4 copies of Lurrus of the Dream-Den you won't be able to open any new ones until you've collected every other card in the set.
You can check out the full list of these Multiverse Legends cards, as well as all of their alternate arts, over at the official website. Similarly, if you're curious about what the Battles and their support cards look like, you can find what you seek by hopping to the March of the Machine card gallery.
The final thing worth mentioning is that the March of the Machine update will finally be adding the ability to select your favorite card style! As of time of writing the system doesn't seem to be active yet, but given that it's displayed prominently in the patch overview, I can only imagine it's going to be turned on once Wizards of the Coast is sure the servers aren't going to explode from the new update.
[Update]: After testing the new system a bit more, it seems like you can only select your favorite card stlye for cards that have different printings. Whether this is a bug or if the system is intended to be this absurdly limited, I'm not sure, but I can only hope they'll extend it to all card styles in the near future as it's quite silly how annoying it is to manage all of various cosmetics.
Whatever the case may be, I hope you have fun with all of the new cards, and I'll leave you with the cinematic trailer: