[Update #2]: MTG Arena has banned Tibalt's Trickery and Memory Lapse from the Historic format, as well as changed some of the digital-only Jumpstart cards.
[Update]: Innistrad: Midnight Hunt has now landed alongside the yearly Standard set rotation.
After being delayed for a few weeks due to technical issues, MTG Arena's Jumpstart: Historic Horizons expansion has now finally been unleashed. Historic Horizons brings with it 782 unique cards to collect, 372 of which are entirely new to MTG Arena and bring with them the potential to create brand new deck archetypes.
It's also worth mentioning that 31 of those new cards are designed exclusively with online play in mind, and as such they contain keywords and abilities that are simply impossible to handle in paper formats. Here's a brief rundown of what to expect:
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 doing the conjuring with, even if there is a bit of randomness in a few cases.
If you're curious how all of these mechanics translate into actual cards and abilities, you can check out the full list of digital-only cards over at the MTG website. As for the Jumpstart packs themselves, the contents of those you can find over at one of the earlier previews.
The official list is unfortunately a little bit convoluted, so if you end up being as confused as I was I'd recommend checking out DetroitKooky's custom made spreadsheet. That one very clearly highlights which packs offer specific rares and mytics, which should make collecting the set a lot simpler and easier.
Jumpstart: Historic Horizons will be sticking around until October 14th, so you should also have plenty of time to dive into the queue and collect your favorites. Have fun, and don't forget that Historic Brawl is now a permanent format as well!