Once Upon A LOOOONNNNG Time Ago...

[Longstorm]

Abyssclan drove Moonclan out of the Forest. HAH!

Through a plan so cunning, a murder most foul, and a spy so malicious, Abyssclan was able to rise against all odds and chase away a Clan of cats who's warriors numbered over thirty! Moonclan was prided to be the largest clan in the forest... You know, before they were driven to the Barren Lands.

Here's how it went: Way back when, when Abyssclan still lived in the City, they sent out cats to all four clans. These cats were trained in the art of deception (being descendants of Boulder) and they inserted themselves into all four clans.
Now, the spy in Moonclan is still there (it's a job that's passed down through the generations). MWHAHAHAHAHA!!!!