Tyrek was the bastard son of a woman in the employ of Casterly Rock.
Tyrek served as a cup bearer at Casterly Rock until Damon Lannister found him work as a stable boy.
After the murder of his mother, Tyrek fled Casterly Rock on a stolen horse and made his way north. On his way through the Oldstones, three members of the Burning Hand's Men waylaid him and stole his horse. Having nowhere else to go and no other options, Tyrek opted to join them rather than turn south and face the punishment of the law.
Near the end of the third era, Tyrek witnessed the death and capture of nearly all the men at Oldstones by a group of Lannister soldiers led by Arthur Swyft. His mentor and friend Tanglefoot Ben helped him hide, and was then killed in front of his eyes.
The fourth era found Tyrek travelling north under the name 'Sand' with a red priestess named Rohanne, a knight named Orson Blackwood, and a sellsword named Morrec. It is unsure whether he is with them willingly or as a captive.
"The men had taken to calling him Sand, after the colour of his hair. When they had left the stream behind the large man had grunted that he “looked the bastard sort” and the name had stuck." - Blood and Whispers
"He was young, only a boy really, with a mop of dirty blonde hair and a harp strung over his thin shoulders." - Blood and Whispers