Thralk was a Dead thing encountered by Sabriel on Cloven Crest in Chapter 6.
Its first death had been the result of a badly cast hunting spear which clipped its throat. It was a resident of the Fourth Precinct for 300 years until Kerrigor returned to life, allowing easy access through the broken Gates.
It chose to inhabit a recently vacated body near to where it emerged in Life and gave itself the name Thralk, easy for a partially-decomposed mouth to pronounce. It did not remember what its original sex had been, but its new body was male.
Thralk preyed on the settlements in The Borderlands for an indefinite period of time before encountering the Abhorsen Sabriel. It mistook her for an inexperienced Charter mage, her diamond of protection being weak, and planned to feed on her. Sabriel, however, caught it with a stop-thrust of her sword before banishing him with Saraneth and Kibeth. When Thralk realised that she was the Abhorson it regretted attacking her.
It was from Thralk that Sabriel first heard the name "Kerrigor."
It has been suggested that Thralk was the spirit of Touchstone's father, as both died in the same manner.