Seath is a white dragon without scales, granted the title of Duke by Lord Gwyn for his assistance in defeating the Everlasting Dragons. Seath was also given a fragment of a Lord Soul along with his dukedom.
As Duke, Seath became obsessed with his experiments and research. He amassed countless tomes and was eventually driven mad by the research. He had also acquired the Primordial Crystal; a sacred treasure pillaged when he turned on the ancient dragons, granting him immortality and making him a true undead.
According to the description of the Moonlight Greatsword, obtained by cutting off his tail, Seath is said to be the grandfather of Sorcery. This makes him the object of Big Hat Logan's obsession. Seath's creations include the Pisaca, Moonlight Butterfly, Crystal Golems and Crystal Ember:
Soul of the Moonlight Butterfly
The butterfly's soul is a creation of Seath the Scaleless.
His servants, the Channelers, assists him in his research by kidnapping women to be experimented upon:
Robe Of The Channelers
Robe of the Channelers, sorcerers that serve Seath the Scaleless.
Even after the onset of Seath's madness, the "snatchers" as they were often called,
ventured to far lands to find suitable human specimens.
The Pisaca are the result of one of these experiments. The humanity that drops from them confirms that they were once human. Two non-hostile ones are found in the tower, crying, who are implied to have been Gwynevere's maidens, for they drop the Soothing Sunlight and Bountyful Sunlight miracles:
Special miracle granted to the maidens of Gwynevere, Princess of the Sun.
Seath is said to be blind, and it is implied by the Six Eyed Helm Of The Channelers description that he sees through his Channelers. In this way his influence stretches across Lordran and other lands:
Helm of the Channelers, sorcerers that serve Seath the Scaleless.
The six eyes arranged in two vertical columns compensate for Seath's lack of sight.
The Vinheim Dragon School is likely founded upon the following or worshipping of Seath and his sorceries.