Complex sentence: Those that do know of the existence of Icewind Dale usually know it for the scrimshaw of the knucklehead trout, a fish unique to Icewind Dale's three lakes.
Split into simpler sentences: Those that do know of the existence of Icewind Dale know it for knucklehead trout scrimshaw. The knucklehead trout is a fish unique to Icewind Dale's three lakes.