Cupcakes

In January 2011, the twelfth episode of the new My Little Pony cartoon was released. Among other things, it featured a light, peppy song about baking cupcakes.

This would lead to the creation of a highly gruesome piece of fanfiction that would become one of the most notorious in the burgeoning fandom, shaping it for months to come.

Plot
Rainbow Dash goes to help Pinkie Pie make cupcakes. Pinkie then drugs her and tortures her to death.

That's pretty much it.

Notoriety
Why Cupcakes became so infamous is strongly a matter of timing. It was released while the fandom was still centered on 4chan, and disseminated from there. For a fandom which had not experienced much or any serious horror or dark fiction yet, it was horrifying--and so despite its generally poor writing quality, Cupcakes became a legend.

Within a month of its first appearance in early February, its fans had cleaned up the writing and released no fewer than fifteen spinoff endings, and the line of Cupcakes-inspired fics stretches in a bloody and disturbing swathe down the fandom to the present day.

Mission and Content
The original and edited Cupcakes stories as well as a slew of spinoff material can be accessed from here.

Cupcakes was handled by Agents Fritz Sorgebrunnen and Silas shortly after their return in late August, as their sixth mission. That mission log is recorded here.