So, the plot begins with me getting Drizzle the Shadow Fiend perk. One night, he's high up in the treetops near the ShadowClan border. He spots a pair of warriors testing their limits by, on their end, jokingly putting one paw over the border while making mocking comments about ThunderClan.

Drizzle, being the cat he is, becomes angry at this. He drops down straight from the trees, landing atop one of the cats and breaking their shoulder with the force. After leaving both cats with some nasty wounds to show, he leaves. All they say upon returning to ShadowClan was that a ThunderClan cat dropped on them from out of nowhere; given that it was nighttime and the area was heavily shrouded in shadow, neither got a good look at their assailant. They both admit to being the ones in the wrong.

As these things often do, the whole situation swells into a rumor that there's a cat in ThunderClan who has mastered both climbing and lurking in the shadows. The rumor swells as it passes from cat to cat, eventually turning into a huge rumor that one of ThunderClan's warriors is better than any ShadowClan cat at masking their presence in shadow.

This rumor gets further fuel when Drizzlenight threatens a group of WindClan/RiverClan apprentices who stray too close to ThunderClan's border for his liking. He speaks from the trees, shrouded in shadow and virtually invisible to them. One of these apprentices hears of this "boogeyman" rumor from a ShadowClan cat at a Gathering, and thus the rumor begins to spread to other Clans.

One night, a pair of cats decide to try tree climbing for some fun. As non-ThunderClanners, they are highly inexperienced in this area. While play-fighting in one of the trees, the branch one of the cats is on snaps and they fall to the ground. They hit the ground at an awkward angle, breaking their neck and dying instantly. The other cat, wracked with guilt and fear over what their own Clan will think, wipes their scent clean from the area and flees.

Upon finding the body of the dead cat, the cats who find them assume it was this "boogeyman" who killed them. Since this incident doesn't occur near the ThunderClan border, doubt begins to arise whether this cat actually resides in ThunderClan. Fear over this cat being capable of striking anywhere, at any time, begins to grip the hearts of many Clan cats.

I plan on further fueling this rumor with more plots, until eventually revealing that this "boogeyman" is Drizzlenight.