Complex sentence: After the completion of the construction of Boudha stupa, Tajibu prayed very devoutly to become the king of the northern region to disseminate the religion, so he was the Dharma King Trisong Detsen of Tibet in his next life.
Split into simpler sentences: After the completion of the construction of Boudha stupa, Tajibu prayed very devoutly to become the king of the northern region to disseminate the religion. He was the Dharma King Trisong Detsen of Tibet in his next life.