Deep Sea Dragons, as their name suggests, spend most of their time in the darkest depths of the sea. They have a bioluminescent dorsal spine that serves to attract prey and communicate with their own kind. As with many deep sea animals, they generally eat whatever they can manage to bait. They rarely leave the ocean floor, surfacing only during breeding season. When they do choose to travel to the shallower depths, they avoid bright lights and will only come up far away from shore.

Dragons are highly-intelligent reptilian creatures that—from a human perspective, at least—appear to live forever. Many different varieties of dragon exist, each with their own unique qualities, habitats, and behavior. Adolescence in dragons is usually marked by the growth of a hatchling’s wings, although not all breeds of dragons grow wings and some breeds have other traits that indicate the beginning of maturation. In Galsreim, dragons and humans coexist peacefully.

After Brendan put Oceantio into the saltwater bay outside his cave, the Deep Sea Dragon disappeared underneath the waves and didn't return. At first, Brendan thought Oceantio had abandoned him altogether, then Brendan learned that the bay was deeper than he thought, and that Oceantio took up residence in these waters. Brendan hardly ever sees the solitary dragon much, though when he does come up every now and then, Itsaso is flying around close by. Brendan assumed that the two had developed a friendship due to their personal place out at sea. Brendan has only seen parts of Oceantio in recent days, but those parts have gigantic proportions compared to what he saw previously. There is no doubt that Oceantio is by far the biggest of all the dragons Brendan has raised.