I had to work to figure this all out I swear- but basic story I came up with is this.

Birchface is a trans Tom, and along with Mapleshade and Reedshine he is wooed by Appledusk. Though he was a bit later then Mapleshade.

She had her kits, and unlike in canon, they passed away young. Presumably before Birchface has his litter of three. When he has them, Otterkit, Shadekit and Hazelkit it’s clear to Mapleshade who their father is.

First she’s upset, enraged, but with the loss of her kits she spins a tale that maybe these kits are to take the place of her litter. Instead of Frecklewish being the overbearing one- it’s Mapleshade. She’s desperate to become their mother. Birchface is accepting of it, after all he understands. He’d probably be the same if he lost his litter.

After that I’m having a bit of a harder time coming up with the rest of the story. Mapleshade probably manages to also lead at least a part of the litter to their deaths. Hazelpaw who would’ve ended up Hazelthroat blinded by Mapleshade and left for dead (she probably refused to follow Mapleshade) and was renamed Deadeye. Her sister and sibling killed due to Mapleshade’s next actions.

Birchface horrified he ever let a cat he assumed to be his friend close to his kids, and Appledusk like canon refusing Mapleshade. And thus canon takes place of Mapleshade hunting down Clan cats.