Vengeance Quest

Vengeance Quest is a popular Goodfic in the Redwall fandom, written by Snowspine (a.k.a. Sounasha or makhsihed). It details the adventures of the squirrel heroine Riala Goldentail. The story is an example of how an author can improve over time; Riala started out as a decidedly Sueish RPG character, and was slowly developed into a well-rounded and interesting character. Notably, every single character in the fic is an OC, but the author manages to avoid Mary Sues.

It can be found here; it should be noted that the story is rated R for scenes of violence, non-graphic sex, and some all-round disturbing content.

This fic is the origin of Agents Stormsong and Skyfire.