The One Oak is the Season Avatars' estate near the middle of Challen, on the bank of the Chikasi River. The house is over three hundred years old, as it was built soon after the Fip Annexation of Challen. The house and property are held in trust for each new generation of Season Avatars.
The house is built around a central atrium where the Summer Avatar grows plants. (There's also a greenhouse elsewhere on the estate for plants that require more sunlight and warmth.) Each side of the house is a wing named after one of the seasons. There are public rooms such as parlors, studies, and a portrait gallery on the first floor, bedrooms for the Avatars and their families on the second, and servants' quarters and an attic on the third. Gardens and a small oak forest surround the house.
If the One Oak is in the middle of an oak forest, why is the house referred to as the One Oak? Just as the Fall Avatar bonds with a special animal to enhance her magic, each Summer Avatar grows a tree from an acorn and bonds with it. The tree helps extend the range in which the Summer Avatar can use his or her magic. When Chaos Season occurs, the Season Avatars rush out to the Summer Avatar's tree to use its magic. (Season Avatars tend to spend most of their time on the estate to remain close to the Summer Avatar's tree. Sometimes when they do have to travel, the Summer Avatar will bring a sapling along to help him/her tame Chaos Season.) Each tree is in use for only a few decades, but they continue to thrive long after their Summer Avatars die and are reincarnated.
Because the One Oak is the Season Avatars' primary residence, it appears in Scattered Seasons, Chaos Season, and Fifth Season. To learn more about the link between Summer Avatars and their trees, read Chaos Season.