This might be "The Storm King" by Warden1207, also published on http://wardenreading.com (I assume the URL works. I'm on a work computer, and they consider that site to be entertainment, so I can't see it).
Leon and his father, Artorias, are the last remaining scions of a once powerful and illustrious family of lightning mages. After barely surviving an attack that destroyed their home fifteen years ago, they now live in the dangerous Northern Vales, a desolate wilderness far away from civilization. But those who want them dead are strong, patient, and relentless, and it is only a matter of time before they find the two they seek...
It involves a boy and his father in a compound in a vale of "The Forest of Black and White" where they build their physical prowess as mages by breathing in the magic and letting it seep into their body. Three chapters in, they haven't traveled into town yet, but I have seen some reviews mentioning that the Spirit Realm becomes important.
Here's an excerpt from the first chapter:
One of those places was the Forest of Black and White, which took up the entirety of one of the smaller vales. No tribe had ever lived there, and few even dared to approach the mountain passes that led to it. It was a place of horror, of terrifying bedtime stories told to the children of the tribes. If they were not brave against their enemies and true to their friends, the wraiths and evil gods of the forest would come to take them in the night.
The vale was one hundred miles east to west and seventy miles north to south, and the Forest of Black and White covered almost the entirety of that land. The forest received its name from the trees of dark bark and brilliant green leaves, and the trees of pale white bark and deep, twilight blue leaves that are most commonly seen within it.
Many strange and otherworldly beings called the forest home, from the vicious beasts of flesh and blood to the spirits of ice, earth, and wind. But perhaps the strangest beings that lived in this forest were two men, a father and son pair, who built their home deep within the eastern reaches of the forest in a large clearing of purple grass.
The father was tall, well-built, and looked roughly in his mid-twenties—though he was at least a decade older than that. He had black hair, warm brown eyes, sharp features, and a straight nose. The son was a youth of sixteen, slightly shorter than his father and with a skinnier build. His features took after his father for the most part, except for his bright golden eyes.
The son, Leon, was dressed in a loose-fitting shirt made of dark green woven grass—a specialty of the nearest tribe—with brown leather pants and boots. He had a hunting bow slung over his shoulder, a quiver full of arrows on his back, and a knife at his leather belt. His father, Artorias, was largely dressed the same, save for a light brown fur coat and a longsword at his hip.
Found with a search for wuxia "forest of black and white"
