And Pitiless As The Sun

And Pitiless As The Sun is a short story by BuffaloBrony. It is technically a prequel to An Old Guardspony's Last Duty, but much of its plot is not directly related.

Summary
Princess Celestia is woken up by her guards with an urgent message: Ponyville appears to be on fire. She finds a scroll that had been magically sent to her while she was sleeping, and the scroll confirms it: Ponyville is under attack from creatures from the Everfree Forest.

When Luna, Celestia, and the majority of the Royal Guards finally arrive, much of the town has been burned to a crisp. An exception is the library, which Twilight had fortified against fire and which remains standing. Inside, the Princesses find Rarity, the Cutie Mark Crusaders, a few other ponies, and Spike. The dragon dies soon after their arrival, having sacrificed himself so no creatures would reach Rarity.

A shocked Celestia is told that Twilight used the Royal Canterlot Voice to order the townsfolk to safety in the library, and then engaged the monsters from the Forest in order to lead them away from the survivors. A guardspony finds Twilight's dead body on the edge of town.

Standing over the corpse of her former student, Celestia blames herself: for not being awake when Twilight sent the letter, for not coming to her aid sooner, for teaching her a particular spell that Twilight used to kill a great number of monsters (and herself).

Lord Cernunnos, a manticore and the leader of the forces that had overrun Ponyville, reveals himself to the ponies to parlay. He had once been a lieutenant of Nightmare Moon's, and had received dominion over the Everfree Forest in exchange for giving up the fight after Luna's banishment. Cernunnos claims that Fluttershy broke the ancient peace agreement between him and Celestia by venturing into the forest and "stealing a life", as he puts it without going into detail.

Celestia does not believe him, and despite Luna's pleas to remain calm, she angrily breaks parlay. In the ensuing battle, the manticore is killed, but so are many of the guardsponies. Their leader dead, the creatures from the Everfree Forest start to swarm into Ponyville to take revenge. Overwhelmed by guilt and rage, Celestia taps into the sun and unleashes a powerful spell that swashes over the monsters. They are instantly disintegrated. So are the surviving guardsponies and Ponyvillians, as the shock wave spreads. The immortal Princesses are the only ones who remain standing amidst "a graveyard of heat and ash".

Celestia faints. When she wakes up, she does so in her bed in Canterlot. The preceding events had been a very vivid dream. They could not have been a vision of the future, since Cernunnos, while real, had long since perished. Celestia is nevertheless unnerved; she has had many similar dreams in recent days, all about her failing to protect Twilight from harm.

She talks about the dream first with Luna, then the Knight Captain of her Royal Guards, Starlight Dancer. After discounting other possibilities for personal or political considerations, Starlight suggests that she send an undercover guard to Ponyville to keep watch over Twilight, allaying her fears. Celestia agrees, and the next night is free of nightmares.

It is implied that the talk Starlight undercover guardspony has with Summer Oak in An Old Guardspony's Last Duty is about that very assignment.