[Update #2]: Wizards of the Coast has revealed the full list of cards coming with the Jumpstart: Historic Horizons expansion.

[Update]: MTG Arena is also permanently adding the long requested 100-card Historic Brawl game mode!

Even though the surprise has been spoiled by an accidental leak, I'm still quite happy to announce that MTG Arena will be adding Jumpstart: Historic Horizons on August 12th. This brand new Jumpstart format will bring with it 782 unique cards to collect, 372 of which will be entirely new to MTG Arena.

While the full list has not been announced just yet, many of these new cards will be fan-favorites from the Modern Horizons and Modern Horizons 2 sets. Perhaps most interestingly, however, Historic Horizons will also bring with it 31 new-to-Magic cards specifically designed with online play through MTG Arena in mind. Here's the sort of keywords you can expect to see on them:

Seek - Allows you to draw a random card from your deck that fits a specific criteria, but without having to shuffle your deck afterwards or reveal your card to the opponent.

Perpetually - Cards with the 'perpetually' keyword can permanently alter the characteristics of a card, regardless of where the card goes. So if a creature targeted with a perpetual -1/-2 effect dies and is resurrected (think Arclight Phoenix), it'll die immediately as its toughness will still be zero thanks to the perpetual effect.

Conjure - Much like Hearthstone's Discover mechanic, conjure allows you to create a fully fledged card (not a token) out of thin air. The exact cards you can receive will depend on the card you're conjuring with, with some of them being quite random and others completely predictable.

Since I know this might still be a bit confusing, I'd recommend heading over to the official spoiler site and checking out the list of revealed cards so far. It should give you a pretty good idea of just what Wizards of the Coast is trying to do with these MTG Arena exclusive cards and how powerful they'll actually be.

If you're more interested in the various themed packs you'll be able to open throughout the Jumpstart: Historic Horizons event, those you can find over at the MTG website. These ones are unfortunately a bit of a nightmare to parse as not all cards are linked properly, but you should still be able to get a decent impression of what the power level of the format will be.

Enjoy the spoilers, and I'll make sure to let you know once Jumpstart: Historic Horizons launches in full.