Blackwater

The sailors and fishermen from Demos are most familiar with these creatures. They often attack ships and even people on land near ocean shores. They are tricky, and won't usually resort to violence directly, but will try and drag you down into the ocean if one isn't careful.
"Black tentacles, to the barnacles." Beware almost completely black looking octopi, as legends in the area say they are a terrible omen to have on a ship. Legends would be right, and also be talking about Blackwater monsters. They can transform into a regular looking octopus to get onto ships mostly without suspicion. They'll wait for night to come and blind everyone, finally striking in the opportunity.
No one knows exactly where they live. Once they dive underwater, they go too deep down to properly follow. Legends also mention something about possible creatures called "Water Beauties" that would sometimes help sailors keep the Blackwaters at bay, but nothing conclusive has ever been found for those specific creatures to date other than some legends.