(This information probably ought to be better organized at some point.)

"Following the disappearance of the Alicorn Brand, the players take on the role of "The Embers" the team of Brand's former retinue protecting the northern kingdom in his absence from various episodic threats. (MHP setting)

This is an early and those lower magic MLP setting, think something similar to Conan the Barbarian."

"Ah, it sounds like the defeat of Discord is fairly recent, within a mortal pony lifetime."

"yes, it's been only a few decades at this point"

character creation:

"Character Creation:
The Embers are not wet behind the ears and each is indeed a legend in their own right. Gathered under Brand's banner during the alicorn's own extensive adventures, they come from just about any region or peoples under the sun.

Aside from general description, characters should have the following:
*Heroic Quality: The dominant attribute that defines them as a heroic figure, courage, prowess in battle, wisdom, magical study, cunning, agility, etc. This should be the way the majority of ponies would describe them in a positive sense.

*Strengths: The three things the Ember most excels at aside from the heroic quality. Example: A mighty warrior might have the heroic quality of Battle Prowess coupled with the Strengths of: Physical Might, Expertise with the Blade, and Incredible Resilience.

*Weakness: At least one thing that is a known or obvious failing or vulnerability to the character. 
Example: Brand has a notorious temper that caused him severe troubles in his youth. It remains a part of who he is even if he's no longer governed by it.

*Deed & Legend: The Embers were gathered in heroic epics and adventures. Each should have at least one major event that created a large part of their reputation (Deed) and the events that led to them and Brand meeting and their being recruited (Legend). These can also be the same event.

Each character has the tools of their trade as well as "The Lord's Brand" a magical mark of their rank and position. Aside from acting as a symbol of authority in Brand's lands, once per adventure a character can call on their magic for one of 3 effects.
Starfire: The pony produces a single large flare light that can be launched up into the air as a signal. These are always visibly distinctive to the character, allowing teammates to recognize who sent out the signal.
Hearthfire: Brand's single biggest claim to the frozen lands is his ability to use his copious fire magic to repel the cold of the land. The embers can tap into some of this gift at will to surround themselves in a warm glow of energy that protects them from the cold.
Battlefire: Lastly the Embers can call Brands devestating flames into their weapons or magic to enhance them for a short while."

"However, a couple clarifying questions on The Lord's Brand's powers, please:
1: Is that one effect per adventure, or each of the three effects once per adventure?
2: Around how long does Hearthfire last?"

"One of the powers per session and hearthfire lasts long enough. That is, unless the session ends up with a significant time skip in the middle of it, it will last the whole thing."

"Thanks.  Though is hearthfire an area of effect centered on the user, then, so that one member of a team can use it to protect the whole team (and non-Embers accompanying them) while others are still ready to use starfire or battlefire?"

"It's an aura that generally doesn't extend past a meter, with the effect dropping off with distance. There's some room for flexing, magic is still young and wild at this time, but the further it's pushed the less time it will last.

Though a pony skilled or trained in fire magic could probably augment that a bit by using their own power."

"So all of the species from the show would be present, including Gryphons, zebras, diamond dogs, hippogriffs/seaponies, kirin, buffalo and dragons.

There's also Harpies and Aranea, who are original to this setting.

Gryphons are currently largely split into roving war-bands under militant generals.

Zebras along with Saddle Arabian ponies have a pretty developed culture and civilization across the sea in the Farlands.

Diamond Dogs are rarely, but not never, encountered above ground owing to their communities largely being subterranean in nature.

Seaponies/Hippogriffs have have yet to flee to the sea, but they do have access to transformative magic. 

Kirin are still largely isolated and have yet to take on their magical silence.

Buffalo are largely unchanged from the show.

Dragons are currently ruled over by the 2nd Dragon Lord, Ossiberus. There are rumors he's planning to direct the dragons in some kind of hostile action as he has recently been calling many of them to the center of his domain.

Harpies come in least, standard, and greatest varieties. Their head and body largely resemble ponies but with the wings and hindlegs of birds. 

Least harpies are half the size of the regular pony and are vegetarian. They largely live in tropical climates in family groups and have an aptitude for music based magic.

Regular harpies are pony sized and with a harsh and militaristic manner. They frequently live in mountain fortresses.  

Greatest harpies are not available and are built on the same scale as adult dragons.

Aranea are spider ponies. They have access to poison, limited shapechanging magic, and mental magics."

""Their head and body largely resemble ponies but with the wings and hindlegs of birds."
Bird wings replacing forelegs, I'm guessing?"

"yes"

"@Maddie:
Is this world known to have or not have an underdark equivalent?  If it's known to have one, what's known about it?"

"It has a number of underground kingdoms so I'd say close enough. But at this point they'd be pretty rare knowledge and your average pony would likely consider them the stuff of legend.

Not that this would mean you couldn't be a Diamond Dog treasure hunter, but it does mean you could expect ponies to need time to get used to you."

batponies:

In ancient times, before the coming of the windigos, the four tribes of ponies shared their ancestral homeland.  As the earth ponies farmed the land to provide the bulk of ponykind's food, the unicorns worked magic from the crafting of artifacts of domestic convenience to the movement of the very heavens, and the pegasi shaped the weather and mustered heroes and legions to defend the land, the nocturnal batponies watched in the night for those threats that might evade the pegasi and delved into the earth for those minerals desired by the ponies but not yet within the earth ponies' ability to grow.  Sometimes the roles of the fourth tribe would overlap, for now and then diamond dogs would make opportunistic raids of the surface, safe in their tunnels from the militias of the earth ponies, the warriors of Pegasopolis, and all but the most powerful or specialized Unicornian mage-knights.

That they were yet distrusted by the other tribes goes without saying, for in those days no tribe trusted any of the others.  While the other three were interdependent, however, and would remain so as they fled south to escape the hungry ice, the batponies in their delvings had discovered the vast and winding caverns deep beneath the world's surface, untouched by Sun or Moon, isolated from weather above, and saturated with chthonic magics that nurtured life in that abyss where only the dim glows of magical minerals, molten rock, and life itself, whether naturally or from its tools, pierced the elder darkness.

Rather than join the competition for warm and distant surface lands, it was _down_ the batponies migrated, deeper than even the cold of the windigos could reach.  There they lived, in villages amongst the close, humid, luminescent black jungles of the deep or in strange-lit cavern-cities of monumental stone where schools studied magics of unending night and warrior orders trained to wield jeweled blades of magma-forged shadowsteel against both proud and greedy diamond dogs and the Things that dwelt in the yet-deeper shadows or the fire of the earth.  Generations passed, the surface to the ponies of the deep increasingly distant history while those above forgot the fourth tribe almost entirely.

But still they were ponies, and so it was that there came a night when Princess Luna of Equestria in her dreamwalking made contact.  From this, for the first time in a great many years, did batponies again fly under open skies -- and yet, not only had they no home on the surface, welcome as they were told they would be in Equestria, they found that their adaption to the depths was not so easily cast off.  In the old days, Sun had been quite tolerable to them, and they could individually shift to a diurnal lifestyle with as little or much difficulty as a member of another tribe could become nocturnal; now, it was an actively painful hindrance to any batpony lacking sufficiently umbrathurgy ability to protect themself.  Additionally, much of the equipment they'd learned to craft that served them well in the depths failed in the surface day, or sometimes outright failed even in surface night.

Thus, although contact had been reestablished and some trade links were made, there would be no great flow of batponies to the surface -- indeed, most surface ponies might live their whole lives without realizing batponies existed.  The only significant group movement of batponies to the surface was the establishment of an elite section of the Night Guard answering to Princess Luna and supplied mostly with Equestrian equipment; barring a few accidents (because ponies are still ponies even when some of them are leathery-winged and far from home), batponies continue to be born and raised only in the depths, and only a minority of the Night Guard's batponies have chosen to remain on the surface after retirement from active duty.  As Princess Luna has taking a liking to their aesthetic, however, many Night Guard pegasi have illusion magic included in their armor to create the appearance of batponies while they're on duty -- which has had the side effect of further reducing the average pony on the street's knowledge of batpony existence, since even some residents of the capital might "know" that the spooky-bat-dragon-pony-thing look comes off at the end of the shift.

first mission:

""Our tale begins the 7th year since the founding of Hearthhome, capital of the northern lands and fortress of Lord Brand, in this home of heroes grand. That winter, when the storms rose high and the snows froze the land, our Lord Brand, as was his want, departed on a journey to the southern lands, to converse with the Alicorn Princesses there on matters of import. In his absence, as he often has, the management of the lands he has tamed is given over to his friend and confidant Chamberlain Frostway. But though the chamberlain handles the simple matters of the day, protection of the lands falls to Brand's Embers, the mightiest of the northern heroes, each pony a legend in their own right, and in their hooves these treacherous and frozen lands are secure.

The lands are secure but not safe, for the winter is ever ready to claim any pony unwise enough to walk before her as she freezes the land before her. This eve a mighty storm blankets the land, and though none knows what flight of foolish youthful fancy took hold of the mind of Little Spark, Hearthtender and pupil to the mighty Brand, that it should cause him to stray into this frozen night, but take hold it did and his absence went unnoticed until well into the evening meal.

When his absence was realized, great was the uproar, for though Little Spark was held dear to the heart of the Lord of Flame, he was not lacking for the admiration and love of all in Hearthhome. More precious than the greatest of jewel in the mighty treasure hall beneath this very fortress, upon realizing he might be lost to the storm besetting the land, all were rallied to begin the search.

Though many hooves be willing, it is only those brave Embers who can truly risk pursuing him into the wild night. As all of Hearthhome searches within the stones of the capital, it falls to these heroes to find the little colt, that he might be returned to those to whom he is most precious." 

TLDR: You are members of Brand's Embers, the mightiest heroes in the lands north of Equestria so recently claimed by the Alicorn Brand. Whilst the master of these lands is absent his young ward has vanished into a blizzard ravaged night and it is up to the players to find and save him."