Oh my this would make for some good post-celestia/luna rule fanfiction, in the right hands.

@Mr Tutorial
Cadence and Twilight become the new rulers, but as Twilight watches all of her mortal friends die of old age, she is filled with frustration and anger. She does all she can to use her magic to save them, but there are some things magic simply cannot do. In classic Cutie Mark Failure Syndrome, she becomes steadily more obsessed and reclusive, resorting to dark experiments in secret places in an attempt to unlock a solution, no matter how unholy.

Finally, one day many hundreds of years after the last of the mortal Elements passed away, Spike dies. This final loss of a lifelong companion sends her over the edge, and in Twilight's insanity she attempts to shatter the barriers between life and death, tearing apart reality and unraveling the world at its seems. No being in the world, not even Discord at the height of his rule, wields the same amount of raw energy and power as the Princess of Magic. Now, ponies cower in their homes as ungodly terrors the likes of which should never have existed roam the earth with impunity.

There is no death, only unlife. No where is safe, not from her.

Equestria's Twilight has come at last.

(yes I would write this story in a heartbeat if I wasn't already busy writing IWPFT)