Hello everyone! Thought I'd share a story setting/plot idea that has been sitting in my head for a long while.

Princess Luna, being the patron of the night sky, discovers an asteroid swarm directed towards Equis. Unlike most examples of this plot, there is no single huge world-killer, but rather a prolonged group of smaller objects that would bombard different areas of the planet for weeks, if not months. Not even alicorns are capable of shielding the entire world for so long, and there are too many asteroids to divert or destroy individually.
Twilight (well, who else would it be) suggests a solution, however. Under the surface of Equis, there are laylines - great rivers of raw magic coursing through the world like blood through the veins. The places where laylines intersect have large magic potential, but ponies have been avoiding messing with those, because it is impossible for an individual to control this power. Instead, their bodies absorbed whatever magic was radiated off.
But after the latest breakthrough regarding magical conduits, Twilight designs a spellcircuit capable of channeling the power of laylines, and suggests using it to create massive shild generators (dubbed DomeStones) all over Equis, effectively surrounding the planet with a forcefield. Understandings the scale of the threat, all races of Equis work as one, barely completing the DomeStone system in time. However, there is an unforeseen consequence to this plan...
No shield spell is perfect, they leak some of the energy that is being poured into the spell. With individual unicorns, it's negligible, and those who can create a city-sized shield also know how to control the leak. However, the immense power that is being channeled through the DomeStones means that the leak is equally powerful. The circuit itself can hold it, but once it's active, no living creature, no spell or magical golem can approach within few miles without being torn apart by the wild energies. The shield holds, but now they can't turn it off.
The far-reaching consequences of it become apparent in just a few months, soon after the swarm passes. With the shield drawing power from Equis itself, ambient magic levels are fading, along with the power of various races. Due to the Draught, unicorns become noticeably weaker, pegasi start losign some of their abilities, and earth ponies are the least affected. The areas around the shield generators - now aptly called the DoomStones - are oversaturated with magic, giving birth to abominations that would give Discord nightmares. There are rings of land with decent magic level, but they, of course, surround the oversaturated areas, and those who try to live there are under constant threat from the beasts within. The ponykind adapts... but it's not over yet.
The shield, still tinting the skies white, has increased albedo of Equis, meaning more sunlight is being reflected off it, and the world gradually starts to cool down. The climate is a mess, and weakened pegasi are hardly able to wrestle the storms into submission. Meanwhile, other races are understandably angry at ponykind for messing up the world they live in, the asteroid swarm having been overshadowed by current hardships. Griffons and minotaurs, who weren't relying on magic as much, become a greater threat than before. Inside the nation, tensions rise as well, with unicorns being seen as culprits, and pegasi being blamed for not preventing weather disasters which they no longer have power to control. This leads to many pegasi adopting nomadic lifestyle, constantly seeking places of relative calm, where wind wouldn't threaten to drop them out of the sky. Unicorns settle the areas around the DoomStones, deciding that the risk posed by the creatures within is worth being able to use their powers again. The earth ponies endure, as they always did, with their magic least affected by the Draught.

This is the world the heroes would find themselves in. Eventually, the Doomstones would milk Equis dry, and the Dome would dissipate, along with oversaturated zones. However, by that time the world would have become very, very different...