Iocaste (and presumably more of her sub-species) lived in the northern part of Toussaint but by 1275 silver basilisks had been all but hunted to extinction, with Iocaste being the last of her kind. A year before she did have two eggs and a mate, but when the male perished, she cast the eggs from her nest.
Despite this, Iocaste stayed in one general area - never venturing out of it and never harming or killing anyone unless they crossed into her territory. The di Salvaress family, who had lived in the same area for centuries, saw the silver basilisks as a good sign and even went so far as to try and protect them. In 1275, this fell to Borhis, who posted signs all around Iocaste's territory to warn people not to travel through the area and generously compensating the families of those who did trespass and were subsequently killed by the basilisk.
However, even with all the warnings, the merchant guild still sent their caravans through the area as they knew, even if Iocaste struck, the Count would compensate them so it'd be like they lost none of their merchandise (with the exception of now several dead workers). Despite this, some merchants had enough and banded together to hire someone to kill Iocaste.
If Geralt investigates, he learns that some Crinfrid Reavers took the contract and plan to lure Iocaste back to her nest and kill her, causing the Count to plead for them not to and that he'll pay them to leave her be. The Reavers will then let Geralt decide on what they should do.
If Geralt decides to leave her be, the Count will state he thinks Iocaste found a new (regular) basilisk for a mate and hopes maybe it'll form a new sub-species.