Oblivion, structurally and emotionally, is where a good Sad/Dark story should begin - that is, the basic elements are present that define it as such. The absence of a typical story arc draws attention to the deaths of the two featured characters, signalling that the intent here is not to entertain with a story about ponies, but to borrow the characters as part of a more personal levelling with the audience. [Incidentally, this borrowing, quoting, reframing or what have you, is the point of this group].

Sometimes the plot gets in the way. The goal of a story like this is to take advantage of silence and the simplicity of the children's narrative, paring down to the bare facts of the characters, location, etc. to create the atmosphere as a prelude to grabbing the reader by the neck and slamming them against the wall. For those skeptical of this approach, I would recommend The Cough by Ebon Mane, a highly successful, short horror story that achieves maximum Sad with the barest suggestion of a plot.

Oblivion poses Rainbow Dash as the character with whom the reader should identify. It can be argued that she behaves according to canon throughout the story. Initially she is seen enjoying the admiration of some of the other townsponies, then, remembering an engagement with Twilight, heads to the library. During the subsequent exchange with a recalcitrant Twilight, the POV remains with Rainbow Dash. Importantly, this creates distance with the ostensible lead character of the series. Elan is cut off from intellect (Twilight is not herself - "...the emptiness in her eyes clearly stating the futility ..."), and immediately the world becomes less intelligible for Rainbow Dash.

Lack of understanding, of encountering something that fails to belong to the ubiquitous Equestrian optimism, is very problematic for Rainbow Dash ("As Rainbow flew towards her home, unease began to form in the pit of her stomach.") Failure to understand is a form of isolation, which as we know she hates. Through the friendship of Cloud Chaser, she decides to act on her apprehension, though still innocent to what is happening: "Still confused, Rainbow reluctantly flew up, heading for Twilight’s house ..."

That perfectly normal, canon-like scene contrasts wonderfully with the next. "Twilight Sparkle lay prone on the stone floor, a pool of dark red blood beneath her." Yes. The word 'suicide' is admirably not used, and the details of the death are not provided. They are unimportant relative to the fact that Twilight Sparkle, the once-vibrant leader, with her indomitable will to know and understand, to explore the farthest reaches of her ambition, has cast everything away.

Spirit cannot remain where there is no will. Meaning is irretrievably lost to Rainbow Dash, whose main character flaw - self-absorption - has perhaps led directly to her friend's demise:

"As she uttered her last words, a sharp pain emanated from her chest. With a quiet yelp of pain, she fell to the ground, writhing for several moments.

As Rainbow Dash exhaled her last breath, she looked into Twilight’s empty, dead eyes.

“I wish I could have helped her..” she thought, her final and only regret as she entered the peace of oblivion."

Just one violent death is enough to break RD's heart, which is just as it should be. She has had no occasion to develop resistance to tragedy. Likewise it is doubtful she had any chance of helping Twilight, having no experience with real depression. Peace here, however, is out of place. There's no peace if Rainbow Dash is dead.

Beyond that, there are a few British forms here and there that would probably not be used by the characters. Also, readers need to know more about the reasons for Twilight's despondency, or it needs to be elaborated on. It would still be within the bounds of showing.

Overall a worthy contribution to the Sad/Dark category, but with room for improvement.