Who should take over the main task of The Elder Scrolls 5 Winter Castle?

Are you looking for Duran, the soothsayer? If so, it's in the dark Bay Mountain. Go in through Bay Hill.

There are two entrances to Beiqiu, one is in the arc corridor on the left side of the gate of Element Hall, and the other seems to be in the noodle restaurant.

Ask a teacher and he will tell you the direction of the entrance.