From Rigpa Wiki
Ba Salnang was one of King Trisong Detsen’s ministers. According to the Pema Kathang, the king sent him to India together with Nyak Jñanakumara to invite Shantarakshita to Tibet. As they met, the great abbot told him that at the time of Buddha Kashyapa they had been the sons of a poultry woman who build the Boudhanath stupa in today’s Nepal. According to the accounts, after helping their mother to build the stupa each of her four sons made an aspiration prayer. The first one prayed that he may establish the teachings of the Buddha in Tibet, and became King Trisong Detsen; the second prayed to be reborn as a great monk to establish the ordained sangha in Tibet, and became Shantarakshita; the third one prayed to be reborn as a tantric yogin to guard the teachings his brothers would establish, and became Guru Rinpoche; the youngest son prayed to be reborn as a minister to coordinate their activities, and became Ba Salnang.
According to Khenpo Jikmé Puntsok Rinpoche, Khenpo Namdrol Rinpoche is an incarnation of Ba Salnang (Interestingly, it is Khenpo Namdrol who went to invite the great abbot Khenpo Jikmé Puntsok to come to India and the West, and attended him making his travel possible).
Ba Salnang is credited with composing the famous Testament of Ba (Wyl. sba bzhed), a chronicle of events surrounding the reign of King Trisong Detsen, the founding of Samyé Monastery and the famous Samyé Debate.