Sea Serpent

Suddenly... the waves had all disappeared. Then the calm ended and the storm began. -Esther, The Adrift.

Sea Serpents inhabit the Arcasinian Ocean, completely surrounding all three continents.
Serpents are the only known creatures to be able to kill an adult dragon singlehandedly. There are several records of distant battles seen from wayward ships from times before dragons became extinct. It is thought that despite the dragons knowing they were outmatched, they were compelled to exact revenge on any sea serpent for the sake of their kind. Unfortunately, this is a major contributor to the extinction of the draconic species.

Habitat & Behaviour

Sea serpents are most commonly found a few miles from any particular coast, since the shallower waters cannot support their needs for survival.
Due to their oceanic habitat, sea serpents are considered Colossal size, significantly larger than Titan-sized creatures on land. As dragons are considered Titan-sized, there was a significant disadvantage. Serpents have been recorded to leap 30 meters in the air. One might think that the dragon's wings would give them an advantage, and many explorers have attempted to fly across the ocean to avoid the serpent's wrath. However, airborne creatures have quickly sent them to a watery grave.
The main way to know if a serpent is nearby is that its sheer size will disrupt the currents, causing rough ocean waves to become calm in a certain area. This is known to be one of the few ways to spot a serpent before it's too late. Sea serpents most likely have vibration sensors running laterally along their body similar to sharks, and some ships have managed to attract the attention of serpents simply by dropping anchor.

Appearance

The Serpent is typically green-blue, with a pale blue underbelly. No actual recording of a serpent's length has been made, but approximations place it at 100 meters in length.
Their head is very similar to that of a dragon, with two horn-like mobile appendages that could potentially be for sensory purposes. The serpent also has auricle fins on the side of the head, pectoral fins at the thorax and pelvic fins in front of the caudal fin.
There appears to be gender dimorphism between the male and female serpents. It appears to be larger auricle fins, and a slightly darker colouring of all fins.