Wikipedia
Sadnalegs or Tridé Songtsen , was the youngest son of King Trisong Detsen of Tibet (reigned 800–815 CE – though various accounts give the beginning of his reign as 797 or 804 CE).
Trisong Detsen retired to live at Zungkar and handed power to his second son, Muné Tsenpo, in 797. From this point there is much confusion in the various historical sources. It seems there was a struggle for the succession after the death of Trisong Detsen. It is not clear when Trisong Detsen died, or for how long Muné Tsenpo reigned. It is said that Muné Tsenpo was poisoned by his mother who was jealous of his beautiful wife.
Whatever the case, both the Old Book of Tang and the Tibetan sources agree that, since Muné Tsenpo had no heirs, power passed to his younger brother, Sadnalegs, who was on the throne by 804 CE.
The other brother, Mutik Tsenpo, was apparently not considered for office as he had previously murdered a senior minister and had been banished to Lhodak Kharchu near the Bhutanese border.
As he was quite young when he came to the throne, Sadnalegs was assisted by four experienced ministers, two of whom were also Buddhist monks. They followed the policies of the previous kings. Sadnalegs had four wives from different Tibetan clans.