He's Just a Murderer - Warrior Cats AU - Short Story

Since the birth of Squirrelflight's kits Jayfeather, Lionblaze, and Hollyleaf, Ashfur has felt nothing but rage. When fire breaks out in ThunderClan's camp, the Clan is scattered. In the chaos, Ashfur finds Squirrelflight alone in the flames, but suddenly wakes up after having blacked out. In front of him now lay Squirrelflight's lifeless body, and around him, the fire still rages. Convinced that only one cat could possibly have killed his love, Ashfur slinks back into the forest to take his revenge.

Warriors series is by Erin Hunter.
Cover art is the Polish cover for Long Shadows.