Songsten was the Abbot at Det-Sen Monastery. He was the only monk allowed into the inner sanctum. He often tried to restrain warrior monk Khrisong from speaking and acting violently.
He was controlled by the Great Intelligence, through High Lama Padmasambhava, in its plans to come to Earth. He brought the pyramid to the caves, accompanied by three Yeti. He ordered the monks to leave the monastery for their safety, but when Khrisong refused, the Yeti killed Rinchen. The monks then prepared to leave under Songsten's directions.
When the Second Doctor discovered the Yeti were being controlled from the monastery, Khrisong went to the inner sanctum to rescue Songsten and Padmasambhava. Songsten, under the influence of the Great Intelligence, killed Khrisong. The Doctor hypnotised Songsten to learn more about the Intelligence's plan. Sapan then took him to safety. (TV: The Abominable Snowmen)