Run, Rabbit Run! By Moonlight You Must Run! (Watership Down)

(Set outside the plot of the TV series)
Sandelford is gone. Obliterated. Destroyed. Ripped apart by the greed and tools of Man.
Aside from Hazel and his ragtag group of escapees, the only survivors were Captain Holly and Pimpernel--who has sadly stopped running-- and they were thought to be the last Rabbits from Sandelford.
But they were wrong. There was another. A doe. One whose fur shines as bright as the first snow with a silver crescent moon marking on her flank.

Moonlight. This is her story of how she reunites with an old comrade and along with Hazel, Fiver, Bigwig, Hawkbit, Dandelion, and the rest take on Woundwort and the terrors of Efrafa