The Under Shadows.

A lot of cats are exiled from their clans for various reasons, treason, Murder, betrayal, breaking the warrior code....
But.
Where do they go?
I can tell you.
There are tunnels, deep underneath the clans, where these cats create their own Clan. They all live there, secretly plotting to wipe out the clans.These cats come out only at night, to steal new kits for members of their clan.
They make holes in the nursery, making it look like the act of a kit eating predator.
But they weren't.
These kits are raised in darkness, with Dark Hearts, and they train for only one thing.
To kill.
These cats train night and day, hoping to get revenge on the clans who had exiled them.