SS is a powerful Necromancer, who seeks to turn herself immortal using the Darkest of Magics. That is, until those annoying Elements of Harmony begin to repeatedly blast Equestria with vast amounts of purifying Magic. Stripped of most of her servants and Powers she goes after the Mane 6.

*** Careful, this story is deeply disturbing and could make some ponies cry. Like Fluttershy, if she's had a bad day.***

Pinkie Pie is pretty blunt so it is barely Teen. Nothing explicit, no Sex and no Clop though.

Spoilers for the Prologue ahead.

This is an account of the Greatest Pegacorn ever. Which is faint praise indeed, because everypony who has been told about the birds and the bees knows that to be the Greatest among the Pegacorns, one would just have to live past twelve. Well with the great magical advances of the last centuries and his own diligent research and studies Princess Stella reached the age of forty, but his time is running out, so he sets his mind to disobeying Princess Celestia and seeking out the people who have a faint chance to help him. They will very probably fail and their Hearts will break, just like the Princess told him, but he doesn't care anymore. He wants to live.

There is an attached tumblr where the Story is mirrored without the Prologue or the Framing device of Diamond Tiara retelling it. There you will find the unedited account of Princess Stella. If you had lived in 2012 you could even have asked him questions there.