The Ashen Wolves are not strictly speaking a noble house, and you won't be able to select them as a faction during the main campaign of the game. They are introduced with the Cindered Shadows DLC. However, you will be able to recruit them in your own house after completing this secondary story. This also applies to the leader of the house.
This house has only 4 students instead of 8, and they reside in a devastated area under the monastery, called the Abyss.
It seems that this house is exclusively composed of fallen noblemen, and they all have a certain predisposition to magic. This makes them versatile and quite fearsome characters considering their statistics.
Ashen Wolves members
It's probably wise to do the main story at least once before starting the DLC, as the Expansion Pass is more difficult and will probably ignore a lot of features and tutorials in the basic game. You will then be able to recruit new students and visit the Abyss freely in your next main game.