The Demon's True Form is the 51st chapter of Tsubasa: Reservoir Chronicles.
Syaoran, Sakura, and Kurogane continue their journey to find the demon. The creatures told them that it lives in a cave-like structure somewhere in the forest. Maybe the demon is something that came from another world, or it could be something form this country that got stronger.
Suddenly, a giant tree starts falling towards them. Kurogane easily slices it apart with his sword. None of them caught a glimpse of the demon. It appears to have some sort of power, but all they have are swords, which means they can only get closer.
Back to Fai and the creatures. Fai wonders what the others are up to. The creatures tell him that the demon is very strong, a strength that is un-approachable.
Syaoran, Kurogane, and Sakura are battling against a strong gust of wind, trying to push their way through. Mokona says that the feathers' presence is very strong. The wind picks up again, and it seems like they won't be able to approach it. Syaoran notices a part of the forest where the trees are not blowing. They continue to approach it, but the wind just keeps getting stronger.
Sakura then tells them that the wind fluctuates form stronger to weaker during certain points. Syoaran grabs her and uses his sword to cut through the ones until they finally reach the cave.
Syaoran realizes, what if the demon was something that is not living, something like a tornado. The wind was not made to keep them away, the was just a force of nature. They are in the eye of the tornado, but there seems to be no sign of the feather.