Sorcerer's Isle
This island in the middle of the Lake is well known to be inhabited by a powerful and very dangerous mage of some description. He seems happy to let the people of the surrounding lands use the Lake, but anyone setting foot upon his island is very quickly killed. Either the sorcerer himself appears and calls down fire and lightning upon the trespassers, or some of the strange beasts who are known to wander the island simply devour the foolish ones who have stepped foot upon his lands.
Fishermen often report seeing all manner of weird and wonderful creatures wandering the island shores, but never have any fishermen been known to have been attacked, as long as they stay off the island.
All attempts by the surrounding nations to make contact with this sorcerer have failed as all emisaries have simply been slain by the inhabitants. Seeing as the sorcerer seems to have no interest in expanding his territory he is being left alone.
For now.