Wa Shi tells you, "Once upon a time, Dereth was nearly overrun with rats. But through the efforts of many people, most of the rats have been eradicated."
Wa Shi tells you, "They can be hard to kill because one must aim low to hit them. I will take any rat tail trophies you find, but rats are much rarer now."
Wa Shi tells you, "I do know of a small lair of rats southwest of the waterfalls. Do not go if you hate rats."