This is not exactly a crossover, but most of the ideas in this fic came from either Fallout 3 or The City of Ember. This is probably the first post apocalyptic redwall fanfiction ever. (If it's not, let me know, I want to read it.) And the "Brotherhood of Steel" will be making an appearance as the remainder of the Long Patrol. And it was not merely Salamandastron that erupted, but what lay underneath it. A super-volcanic eruption can definitely cause this much destruction, if not more.
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Salamandastron.
Since time immemorial, it stood as a beacon of hope and an ever watching sentinel for the land of Mossflower. The long patrol and the badger lord or lady who lived there watched the sea for signs of an invasion, and brought help to those in need.
All of this changed. The mountain began to rumble. For about a month, Everybeast in the land of Mossflower knew what was ahead, due to low rumbles coming from the southwest. Some had time to prepare, to dig shelters, or to retreat to cave systems, bringing all of the food and supplies that they could carry. Some were not as lucky. Then it happened. Everybeast in the land felt the tremendous blast. Then it came. A tremendous wave of heat and volcanic ash swept over the entire land, not just Mossflower, but Southsward, the Northlands, and the Wastelands, incinerating anybeast who had not found safety.
The sun was blocked out completely, for almost a season, as the place where Salamandastron used to be continued to expel ash into the atmosphere. For a while, the known world fell completely silent. But life endured. And the first beings to come out of safety were greeted with a very bleak sight. The land had become a snow encrusted wasteland. The charred skeletons of trees were all that was left of the once vast Mossflower wood. After a long while, the snow melted. The only plants that could survive were small shrubs and grasses. In some places, the ground was completely black, charred by the intense heat of the eruption.
Vermin also emerged. Bands of raiders and thieves roamed the land with fee reign. The charred remnants of almost every single farmhouse in the country were robbed of almost everything. The land had been thrust into complete anarchy. Then a ray of hope emerged. A ship emerged on the horizon, filled with the hares of the Long Patrol, hoping to bring peace and civilization to the ravaged land.
This story starts a mere two weeks before the eruption, at Redwall, as the woodlanders prepared for a migration to the one place big enough to hold enough supplies for months, and enough room to accommodate all of the abbey creatures: The ruins of Malkariss.
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This is only the prequel chapter, so don't think this is the end of the story. Rate and review!!! And I know that Vermin are already raiders and thieves, but I just wanted to emphasize the point that woodlanders weren't the only ones that had survived.