From Tar Valon Library
Kerb was an apprentice chandler, who, acting under Graendal's compulsion poisoned the messenger she had sent claiming to be from Alsalam when it looked as though Rand might be able to trace her using him. Nynaeve apprehended him and under Rand's instruction removed the compulsion, which was the only thing keeping him alive. He died, but was able to first reveal her location.
(Reference: The Gathering Storm, Chapter 32).