[Update #2]: The Wilds of Eldraine set has now arrived with a bunch of new cards, improved duplicate protection, as well as Alchemy rotation.
[Update]: MTG Arena is getting Khans of Tarkir in 2023, while Modern Horizons 3 and Pioneer Masters will be coming in 2024.
After testing the waters with two Dungeons & Dragons themed expansions, Wizards of the Coast has now finally decided to take the plunge by releasing a sizable crossover set based on J.R.R. Tolkien's The Lord of the Rings. For MTG Arena this means that there's a bunch of new cards to play around with in the Alchemy and Historic formats, a fresh Mastery pass and the usual assortment of cosmetic goodies, new Jump In decks to smash together, and naturally, the full suite of limited events like Draft and Sealed.
Since Tales of Middle-earth is supposed to be an introductory set and an onboarding tool for new players, there aren't that many new mechanics to talk about. In fact, the only noteworthy addition is "The Ring Tempts You". Whenever this happens you'll have to choose a creature as your Ring-bearer (if possible), as well as gain or upgrade The Ring emblem that will grant your Ring-bearers special powers. These powers are gained in sequential order and they all work together, so a fully upgraded emblem will grant your Ring-bearer all of the following abilities:
1) Your Ring-bearer is legendary and can't be blocked by creatures with greater power.
2) Whenever your Ring-bearer attacks, draw a card, then discard a card.
3) Whenever your Ring-bearer becomes blocked by a creature, that creature's controller sacrifices it at the end of combat.
4) Whenever your Ring-bearer deals combat damage to a player, each opponent loses 3 life.
It's a simple effect, but as many players have already discovered through the various limited formats, one that really should not be underestimated. For example, even getting your ring emblem to rank #2 is enough to give you a massive advantage as you'll be able to ensure you don't end up with too few or too many lands. Quite handy, especially since "The Ring Tempts You" cards all come with additional benefits!
But enough of my rambling! You can check out what the new mechanic looks like in action, as well as what the accompanying cards are even all about, over at the official card gallery.
Once the next big update arrives, most likely once Wilds of Eldraine launches in September, I'll make sure to let you know. Until then, have fun with Gandalf tribal!