On the last season 7 episode 7 the last scene where the Night king's army broke down the wall & started their march towards the seven kingdom..if u noticed carefully, The formation of the Night King's army was look like a 'direwolf's head' which indicates that someone from Stark's family member have done this..As Bran Stark's is the 3 eyed raven & he can control anyone,anytime.So It's not impossible for him to control the Night king's army as well as the knight king!!!! To know the reason behind this u have to wait till season 8 which will be release in 2019...
