Thralk is a Dead creature encountered by Sabriel on Cloven Crest. He had been a resident of the Fourth Precinct for over three hundred years until Kerrigor returned to Life, allowing easy access through the broken Gates in his violent wake. When Thralk emerges in Life, he enters a recently killed and already rotting body.
Thralk preys on the settlements in The Borderlands for an indefinite period of time, as he needs the life-force of humans to stay out of Death. When he encounters Sabriel, she banishes him with Saraneth and Kibeth.
It was from Thralk that Sabriel first heard the name "Kerrigor."