In the crater of an inactive volcano on Arcana, a small sea of shifting colors resides. This magical semi-liquid forms a single collective entity, known as The One. Occasionally, in its search for knowledge, The One sends out an Arm, a singular piece of color from the swarming pool. These singular entities are known to the rest of the world as Seaborn.
An individual Seaborn is approximately three feet tall and a little wider; their natural form is rather gumdrop-shaped. Externally, they have a clear, thick, malleable membrane. Inside that membrane is a thick magically-charged fluid, and suspended in that is a core, composed of that fluid, but heavily densified to create a sort of brain. It's connected to the membrane by a series of muscular fibrous cords. Seaborn have a pair of dark eyespots on its front that it uses to see. Their natural color variates wildly; each particular Seaborn could be almost any color.
One of their most astounding physical abilities, though, is their ability to change their shape. Using a combination of those muscular connections and their magical nature, they can reshape their outer membrane to match almost anything that they've seen, as long as it's about their size. They're incredible mimics, though they naturally can't change their color. They can maintain a form as long as they can keep at least a little bit of focus on it. They can be startled out of a form, for instance, and when they sleep, they revert to their natural shape.
Seaborn also have the ability to let things through their membranes, when they want to. Their membranes are smooth all around, and as such, they hae no mouths. They eat by engulfing an item of food and absorbing the liquids and nutrients out of it before releasing the solids back out again. They can also hold objects in a similar manner without harming them. However, holding sharp objects, like a knife or a sword, comes with risks; if they get jostled too much, like from an attack or a fall, the objects might pierce their membrane from the inside.
Seaborn cannot speak, or make any other sort of vocalization. However, they have low-grade psychic abilities, as part of their make-up; these are how the many become The One. By physical contact, a Seaborn can reach into another person's mind, sharing surface thoughts and feelings, and - if the person is willing - they may be able to delve into the subject's memory. Others with psychic talents can form stronger bonds, able to share thoughts even without contact.
Over hundred of years, the volcano that formed the continent of Arcana fell dormant, the top slowly sealing and creating a giant bowled crater. Inside this bowl, a lake formed, one choked with living matter too, and over the years, this lake became a sort of primordial soup, swarming with colors. It absorbed the overflowing magic of the continent into itself, and slowly, sentience came to those colors, binding the lake together. Each color was a facet, and they all came together to make one tremendous mind. As the multitude became a single mind, it began to think of itself as The One.
Newly risen, The One had a hunger for knowledge. It began to study the wildlife around it, finding out all that it could about everything that it could. Within years, it knew of and about every thing on Arcana, and it yearned to know more and expand its knowledge beyond the horizons of the continent. As an experiment, The One released a single color from its swirling form. As it left the mass of The One, it shifted to its own shape, and the link to the race's collective consciousness disconnected; this new creature was a sentience of itself. Dubbing it his Arm, The One sent it to the shore and out into the ocean, on a mission of discovery and learning.