"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.
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. The fic's entire cast is comprised of original characters, none of them Sues (though a parody of famous Canon Stu Drizzt do'Urden appears). However, canon locations such as Redwall Abbey, Salamandastron, and Castle Floret appear, and are very important to the plot (the latter is the site of the final battle).
Chapter 1: Riala Goldentail is the main character. Her father, Rilar Battlecry, has sworn to slay the infamous wolverine Nightdeath Longclaws, leader of the Nighthunt horde, in revenge for the death of his wife. Longclaws eventually finds him and challenges him to a duel, but gives his archers a prearranged signal to open fire as soon as Rilar gains the upper hand; Riala herself barely escapes being shot. Riala swears vengeance on Longclaws in turn, and goes wandering. (Note: Due to an error, the page for chapter one actually contains the first four chapters.)
Chapter 2: Four seasons pass as Riala stalks the Nighthunt, making a name for herself by helping goodbeasts and fighting vermin whenever she comes across them. During her travels, she comes across a pair of weasel slavers taking a mole family prisoner, and the sounds of the moles (particularly the dibbuns) being whipped sends her into a rage; Riala quickly dispatches the first slaver by slashing his throat, and is about to kill the second when she is stopped by Soilfree. Eventually, Riala spares him, chaining him up; after a conversation with the mole family, in which she explains why she is seeking revenge, she takes the second slaver outside and interrogates him, trying to learn where the rest of his band is. When she finds them, she frees the slaves, who kill their captors.
Riala arrives at the North Caves (the birthplace of Martin the Warrior), is introduced to the woodlanders inhabiting the caves by their leader Mark the Warrior, and tells them her story. The caves are attacked by the searat ship Blacktooth, under the command of Captain Deathclaw. Riala joins the battle, is sorely wounded, and stays in the caves to be healed.
Chapter 3: Riala leaves the North Caves and travels south. In the Badlands, she is captured by toads and rescued by hares of the Long Patrol. The Badger Lord Firesight welcomes her to Salamandastron, but the hares react with shock upon seeing her tail. It is revealed that a prophecy in the carvings inside the mountain predicts that the arrival of a goldentailed squirrel signals the day the Badger Lord will die in battle with Zarok the Black, a fox warlord. Firesight dies in the single combat, taking the fox with him. Much to Riala's surprise, the hordebeasts surrender honourably and are allowed to leave unhindered.
Chapter 4: Riala wanders through Mossflower in the winter. She is attacked by a pair of rats – Bluddfang and Muddclaw, a mated couple – and slays the female Bluddfang, suffering severe injuries in the process and being knocked out when Muddclaw's grief spurs him to attack harder. When she awakens, she is in the Infirmary at Redwall Abbey, where the healers are astonished she survived and doubt she'll be able to fight again.
Chapter 5: Riala begins to recover from her injuries, and after two seasons is able to walk down a set of stairs, although with the use of crutches and most of her strength. She meets Kaylen, a female otter that found Riala in the snow and brought her to Redwall Abbey, who is offended by Riala's obsession with combat and fighting over living. Riala later apologizes, and the otter both offers to help the squirrel recover and points her to the Abbey's records for any clues regarding Longclaws. The depth of her obsession with Longclaws leads her to threaten and frighten Josiah, the Abbey Recorder, after he tries to stop Riala from reading scrolls until she is willing to treat the papers with respect. Her physical recovery, however, progresses well, and soon she is among the better fighters in Mossflower woods, although still outclassed by the healthier Kaylen. She is offered a place within the Wanderers of Mossflower, an organisation of goodbeasts operating as scouts and fighting aggressive vermin, but Goldentail's unwillingness to recognize peaceful or even reformed vermin puts her at odds with the rest of the group, especially the black vixen Shadow de Vulpes.
Chapter 6: Patrolling Mossflower as a member of the Wanderers, Riala meets the badger male Onestrype Durando, one of the Wanderers' more isolated members. She also finds a good location for a drey, and begins to build a home. The next spring, she even settles down enough to join the Abbey's Nameday feast, and patch her clothes. While she is still cold and withdrawn near Shadow de Vulpes, she does begin to trust the fox, and does not make a scene when Shadow joins the feast a few seats down from her. The ceremony is interrupted, however, by reports of a trio of slavers at the ruins of St. Ninian's, and Riala goes off alone to confront them. She takes two of the vermin down easily, but a hidden ferret archer and stoat swordsbeast turn an easy fight into a hard one, and a black vixen using a slave as a hostage makes even Riala pause. Shadow de Vulpes threatens to cut an enslaved squirrelmaid's throat if Riala doesn't hold back. It is, however, a trick, and the fox cuts down two of the vermin, giving Riala the opportunity to take down the last. Riala adopts the squirrelmaid, who picks the name Malaya.
Chapter 7: Riala and Malaya find Onestrype fighting an unknown wolverine female in a bloody battle to the death. Onestrype kills the wolverine with an axe strike to the spine, but is grievously wounded. Riala binds his wounds and Malaya goes for help, but the Abbey healers are unwilling to say if he'll live. The smell of wolverine brings Riala to a rage and drags her focus back to her quest. Malaya's inability to deal with the blood and gore of combat leads Riala to begin training her in the use of a saber.
Chapter 8: A single stoat in tattered clothing stumbles into Mossflower, wearing the uniform of the Longclaws army. Riala tortures her, breaking paws, eventually cutting off fingers and ears. Before the stoat dies, she tells of the strength of Longclaws' army, the names and skills of the captains, and their next target: a castle in the Southlands. Her last words are "Kiern… I’m so… sorry…". Shadow finds Riala standing over the corpse, and warns her that this type of torture is not accepted. The Wanderers of Mossflower kick her out of the organisation, and she begins to head south to Castle Floret.
Chapter 9: Kiern, leader of the Nightclaws and subordinate to Longclaws, watches three of the captains executed in an attempted mutiny. Wildclaws is injured in the fight, and Stormsong stitches the wound while explaining why vile beasts like Veneno and Deathcry are being promoted to captain. Kiern also reveals to Skyfire that he was rescued by Longclaws from a life of slavery, although not before his sister was sold away.
Chapter 10: The Nighthunt plan an attack on a small group of armed woodlanders. Impatient assassins make the battle bloodier than it needs to be, but the woodlanders are outnumbered and the fight is a massacre. Kiern spends most of it trying to keep Vereno and Astarte Darkmoon from slaughtering too many woodlanders; the Longclaws has plans for the younglings and captured warriors. The warriors are told that their young will be tortured and killed if they do not follow orders: to attack a clan of foxes. If they do as they are told, the younglings will not be harmed, nor does the Nighthunt keep slaves.
Chapter 11: The woodlander prisoners attack a small family of foxes, killing the mother and injuring others, before the Nightclaws charge in and kill the woodlanders. This is how Longclaws recruits: offering rescue and safety from threats he is truly responsible for. The surviving foxes are inducted into the Nighthunt's front line troops, the Nightfangs, although Astarte Darkmoon notices promise from one of the vixens.
Chapter 12: Kiern retreats to slumber, only to be startled out of sleep by screams. Longclaws gave three of his command woodlander captives to rape, and Kiern stops them and assigns them latrine duty on half rations. When he questions the Longclaws, it's explained that the wolverine doesn't think woodlanders count enough as beasts to be raped. They will be sold soon, for although the Nighthunt does not keep slaves, they don't care for woodlanders; the choices are to sell them into slavery or leave them to starve. Kiern struggles with his honour, before Stormsong reports his scouts to see movement, presumably squirrels. Kiern tackles Longclaws to the ground, taking arrows in the process.
Chapter 13: Nearly half a season later, Kiern awakes to the sound of Stormsong's lute. His injuries are still severe, and are slow to heal, but are closing. The Longclaws had given up on any recovery, but Stormsong insisted upon continuing to hope. Kiern begins to train with Stormsong's physical therapy and sparring with Skyfire. Eavesdropping near Stormsong's tent reveals that Skyfire has fallen in love with Kiern, and that Stormsong loves one beast who cannot love him back.
Chapter 14: Longclaws is planning another execution. Stormsong greets Kiern with a kiss, revealing both his tastes and the target of the next execution. The rat Sharsek had already outed him, and Longclaws decides on a particularly horrible period of torture. Stormsong's lute is destroyed, he is beaten, sexually assaulted by females, and raped by a male fox, before he is mercy-killed by Astarte Darkmoon. Longclaws informs Kiern that Stormsong had fallen in love with him, and insists that Kiern sleep with a female stoat weekly; Kiern visits Astarte rather than abuse his position with Skyfire. Skyfire deserts the Nighthunt, warning Kiern that he is becoming cold and emotionless like Longclaws.
Agent Stormsong is recruited from the beginning of this chapter. Agent Skyfire is recruited somewhere between the end of this chapter and Chapter 8.