If Karsa relays correctly, the... tongue... ( Somehow, he never manages to sound unperturbed by it. ) Arrived with Captain Vane to serve him as a compass to the Crossing during two incursions, two years apart. He retained it on his person and appeared... sufficiently sane to begin his latest expedition, contract an escort and, candidly, fool me into allowing you as his passengers.

( Not that the Merchant was especially picky. ) I imagine the trouble there was far more linked to Captain Vane's pre-existing... horrors lived in the Crossing. The... tongue must have latched onto his vulnerability, as expressed in its domain.

In other words, I would not be surprised if we are looking at the distinct situation of an already mentally compromised individual.