A Shadow Among Us follows former Darkkit's, now named Shadow, PoV as he learns about he evil Clans from his mother figure, Fern, his father, Benny, and Fern's mate, Shard. As he grows into a well-muscled, mature tom, he begins to form a plan. He will avenge his mother.
Shadow travels to the Clans, pretending to be a lost loner seeking shelter. He is accepted into RiverClan, where a civil war is currently taking place after the loss of the leader and deputy. Seizing his opportunity, Shadow weasels his way into being the leader of RiverClan. He appoints a deputy and medicine cat who he trusts will follow his every order, and begins killing every member of the Clan.
He goes through the dens every night, killing one cat and dragging their body into the clearing. After several nights of this, he bumps into a pretty silver tabby she-cat while he's dragging the body of an apprentice to the regular spot. He instantly falls head-over-heels for her, and he thinks she loves him back, but at the next Gathering, she announces his crimes to the crowd. Shadow is killed, but not before the silver tabby could whisper in his ear, "I loved you, but we could never be."
(You should probably start making the she-cat notice him early on, maybe with simple gestures like shooting him an admiring glance or bringing him a piece of prey.)
Here's a cover I made for it!