In Fallout: Equestria, the fate of Rainbow Dash is unknown. Of the Mane 6, hers is the only fate that Littlepip does not discover in the course of her adventures. She learns things about her that suggest what happened, but nothing which confirms it.

Tom Cullen, who created the "F:E Fates" series of artworks, made this comment upon finishing the image below:

"A lot of folks were wondering about this one, as Kkat never really addressed Rainbow's ultimate fate. Finally, I chose to go with this scene, our final glimpse of Rainbow Dash before she faded from the story, and from history... I know it's weird, having a Rainbow-shaped hole in the history of the Equestrian Wasteland, but I actually prefer not knowing. It's kinda poetic...As hard as Rainbow worked to make history, the break every record, to have her name screamed to the heavens, it somehow seems right that in the end she should just...vanish."

Art by Tom Cullen

However, while the story did not tell the final fate of Rainbow Dash, I certainly had my own headcanon for it. And earlier this year, on my birthday, I revealed this headcanon to several others and fans in an IRC chat, along with part of my headcanon for the Crystal Empire (as seen in my Crystal Empire Blues blog posts). For those who are interested, you can find a log of that conversation here.

Please bear in mind that this is strictly headcanon, and not to be taken as actual canon. It should not interfere with anyone's stories or replace a headcanon that any reader prefers. In fact, I tend to agree with Tom Cullen that Rainbow Dash's story is best left as Fallout: Equestria left it.

It has come to my attention that at least one FoE side story author has chosen to draw upon this headcanon in the crafting of their own story. And that, in turn, has inspired the creation of an extremely impressive and delightful new brony song. Check it out!