A returning mechanic.
A mechanic that makes the card it is on cheaper to cast.
A mechanic that works with counters.
Remade Bloodthirst.
Remade (and fixed) Cascade

Resistance is a new ability word that is very similar to Bloodthirst. It also makes the cards it is on stronger if an opponent was dealt damage the turn you cast it. However, while Bloodthirst could be put only on the creatures, Resistance can be put on any card type, if it's on a creature, it doesn't need to trigger when it enters the battlefield and doesn't need to work with +1/+1 counters. In fact, there isn't even any card with Resistance in the set that puts +1/+1 counters on a creature.

Take Fiery Revenge, for example. It deals 2 damage to each opponent normally. If an opponent was dealt damage before, (and it doesn't matter if it was by a creature or some burn spell, for example by another Fiery Revenge) Fiery Revenge deals 3 damage instead.
Now, let's look at example how does Resistance work on creatures. For example, at this one:

Steward of Frontiers. A dwarf with an untapping effect, but just to creatures that dealt damage to an opponent this turn.
Those two cards are commons that appear first in red, and second in white. The question is: In the Thirunea set, does Resistance appear in another colors other than these two?
The answer is no. It appears only in red and white, together with cards that helps Resistance effects to be activated. Similar it is to other Thirunea's mechanics. Every of them is connected to only two colors. And to all set's draft archetypes, one "fraction" is connected.

The last one of today's previews. These cards are all for the Restorer's pact's RW Resistance draft archetype. And what exactly the Restorer's pact is? The Resistance is literally saying that its bearers are rebels, and that is true. But more pieces of information about the Restorer's pact will come later.

Expose is a new action keyword, working as a much worse Cascade. The ability triggers when something happens. Mostly, it's a creature entering the battlefield, dying, or spell resolving.

I will explain how it works on the most basic example, Sneaky Sleuth. When it enters the battlefield, the expose ability is activated. Then you exile the top card of your library. If it's a permanent and it has lower converted mana cost then the card that caused the expose ability to trigger (in this case 1 or 0), you may put it on the battlefield. Note that this will cause "enters the battlefield" abilities triggers, however it won't cause any "cast" abilities trigger. Similary as to each mechanic in the set, this set has some cards to make it better and in expose's case, to make it less random and less about luck.
This ability appears also on noncreature cards:

On Unravel the Heresy, for example.
As you can see, this mechanic appears in green and blue. Now let's look at more complicated card:

Now, this one might be chaotic a bit. If you decide to expose, the newer instance of expose created by Inspired Dirtseeker resolves sooner. This means, because the source of this expose trigger is Inspired Dirtseeker, first card will be put on the battlefield if it's a permanent and its converted mana cost 2 or less. The second will depend on the other source of expose.
This is the last one of today's previews. All of them are for Dirtseekers' Union GU archetype. All its members are some kind of detectives. I will write more about them later.

Firstly, a strictly better Terashi's Grasp.

A little creature that can give later some of your creatures first strike in case of need.

A reprint with a quote by some legend that will appear in the set.

A member of the Clergy of the Fall.

A spell that ensures that some wounds will never heal. Also, it confirms that -1/-1 counters will appear in the set.

A reprint of a card from Ravnica: City of Guilds.

Instant Dragon Fooder, however it has a little drawback.

Spell to disable bigger creatures to block or to kill the small ones.

A combat trick that also creates a token.

Another card with the Expose mechanic.

Now, this one is powerful. Mana Leak, Turn the Tide or an instant creator of a 2/2 Illusion? Your choice. Part of this set's monocolored cycle of Charms.

Really nice creature that also creates some tokens and a very good card for Limited.

Some kind of bizzare counterspell with a looting effect.

A new planeswalker for Thirunea, Arwine, World's Shield. Despite that I didn't have an intention to reveal her so soon, however when I once said that it is a random booster, so let it be.
So, I will now tell you something about her:
Arwine is a merfolk planeswalker whose home is Thirunea. Years ago, she was a member of the Unifiers' council, believing that the Council is doing everything to make Thirunea safe and beautiful plane. However, when she has seen what the government is doing with people who are against it, her Spark iginited. After she returned, she has sworn to protect the people of Thirunea, with everything that won't harm the innocent people. Her first task was to protect her home from the event that was slowly coming - the Fall, the day when all the faithful will die in peace, while all heretics will have to suffer eternally. She created a magic orb called 'Arwine's Fall Delayer' that stopped the Fall's arrival. Priests tried to destroy it, however their spells weren't enough. The orb was guarded by nobody, however whenever one of them tried to destroy the orb, they died in horrible way. Now, all the faithful know that they have to use a different way to destroy it. They know that Arwine created it, and so they are trying to catch her. However, they haven't yet succeeded. Arwine is now hiding far from them, knowing that her presence on the place is protecting the orb. If she would die or planeswalk away from Thirunea, the orb would remain without a protection. And she can't allow that to happen.
A special basic Island.

A token you've already seen.