Future Sight

The new mechanic is Plague. It's a pseudo mechanic that describes an effect that happens when a creature dies if a -1/-1 counter was put on it that turn. It's the closest I could think of to a mechanic which is consistent with existent wording for an effect that is mean to trigger when a creature dies to -1/-1 counters specifically.

Sorry for delay. I don't know why but I totally missed these posts and I thought this challenge was not attractive.

Illuna, Apex of Compleation/pernicious
It is very flavorfully depicting Phyrexian invasion.
It allows you to mutate only once for each creature in most cases.
It is less exciting than mutate but also less problematic on playability and is polished up in total balance.

Ashling the Flamespark/vizionarius
At first, I couldn't get how it works. As now I got it, I feel it is indeed impressive.
Wizards says that MoM will change the Multiverse forever.
And the Mending, the last time it had been changed, was the reason why we can summon PW.
So I am forecasting that something that changes both MtG's mechanic and flavor will come other than battle.
The ability is enough attractive probably for many people.
One thing that bothers me is that it uses two types of counter both are counterable thing.
It might have more playability if it uses only loyalty counters and has "gets +1/+1 for each loyhalty counters on it".

Schemer of Millenia/Idea
It almost assures the win if it could survive a turn after you hard cast it.
It has no immunity against removal spells so I guess the most players who tried to use this card as a finisher would finally exchange it with another one.
But anyway it has an strong attraction which would make many players want to use it in aspects of both ability and flavor.
And there's no doubt about that it is very strong once it survives a turn so it is possibly a good finisher for some decks.

Winner
Winner is vizionarius .
Once I hesitated to give Ashling the win because the mechanic seemed to be limited for rare or mythic and not suitable for the feature of a set.
But if the ability appears in uncommon like WotS, it can be said the feature of a set.
The zero initial loyalty and weakness of being both a creature and a planeswaler seems to be easier to balance it for R&D.
The deciding factor is that it finely work with proliferate.
the runner up is pernicious. The compleat is flavorful and can be used for common cards and perfectly matches with Phyrexia.
I like Idea's one as the card itself for I am a Bolas fan, but I doubt infect will be beck at now we have toxic, which is a tuned infect.

Thank you for your participation.

Bump for entries. I'll add to the prompt to help it along.
Design a Battle that would fit on Eldraine.

Edit: Note, it doesn't have to be a Siege. Make up a type and mechanic for a non-Siege battle card, if you'd like. I'll give this a week, and if nothing comes along, I'll switch the prompt completely.

Sometimes a more directed approach helps people with being stuck in coming up with things. My understanding is that this thread is meant to be speculative. So whatever you come up with is fine.

Judging tomorrow.

Feedback

ameisenmeister: This is a really interesting effect! Leads to quite a dilemma of a choice that we haven't really seen, so feels like a new direction. Also fits in with the fairytale feel of having to choose between two paths. Your card specifically has a nice mirroring effect. If they choose a lower number than what you care to draw, then they get hit with being set back 3 cards, but it also gives them a bluff with tempting you with 5 cards. Just a note that you never define X, but I assume it's just the chosen number.

Idea: Torby the Crackguard is very strong! First strike really puts him to a bit of a broken place for me. What is essentially a 5/5 vigilance for 4 who can block as many creatures as he wants is already strong. Adding first strike is a huge icing on the cake. But that he can block three 3-toughness things without even getting hurt is quite a boost. With just a bit of pump to his power, he can solo a whole attacking army! Though I do like the story here, and the edited art looks great! I might drop the first strike personally to cool his power a bit, but I like what you did here, even if he is pushed. (Note that I can see how there may be a case made that since Phyrexian Obliterator and Phyrexian Vindicator exist and are very strong too, that Torby is not too strong, but they can't be commanders, and they don't prevent an opponent from being able to attack without damage going through, which I think sets Torby apart.)

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Result

The winner is ameisenmeister with an interesting new mechanic and one that fits well with the general fairytale theme.

Ferocious Raptor 
Creature - Dinosaur
Menace, frenzy 2 (Whenever ~ attacks and isn’t blocked, it gets +2/+0 until end of turn.)
Whenever a creature dealt combat damage by ~ this turn dies, put a +1/+1 counter on ~.
3/2

Thanks or the quick hand-off win, ameisenmeister!

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Next: What do you think will be the next Universes Beyond complete set, such as Lord of the Rings was? Make a few cards showcasing some of the major mechanics that would be in that set.

would be great to see at least another entry!

ameisenmeister: While unfortunately I know very little about Star Trek, the little I do know comes through your designs very nicely. I really like establish as a mechanic! Focus is also a really neat way to have cards "charge up" for something big! Love the Vehicle theme and the allusion to factions on the Charm. Sagas are a great way to show off parts of stories, and you do that excellently with your one here. I imagine there would be a set of premium versions of cards with Klingon translations on them like how we have the Phyrexian ones, except they'd have meaningful and speakable words on them!
Too bad this thread hasn't been more popular. I would have loved to see cards from different franchises. If you want to make the next challenge, go for it!

Alright, as our next set will be Wilds of Eldraine featuring an enchantment focus, please design a creature enchanting aura that that does something innovative. Think Emblem of the Warmind or Daybreak Coronet.

Have fun!