Seath the Scaleless

Seath the Scaleless is one of the bosses and a major antagonist in Dark Souls.

Envious and dissatisfied by the fact that he was born as the blind mortal dragon, he chooses to switch sides with Lord Gwyn and sought alternate means to be immortal for centuries. He eventually met his downfall after his Primordial Crystal was destroyed by Chosen Undead, the protagonist of the game due to the reward of his cooperation with Lord Gwyn, Bequeathed Lord Soul Shard, gave him odds with the hero.

Background
Seath is an albino dragon who was born without scales. Not having scales means he lacked the immortality that all other dragons possessed. Out of jealousy of the other dragons, he betrayed them, siding with Lord Gwyn in the war against his brethren. After the Everlasting Dragons were wiped out and Gwyn emerged victorious, he granted Seath the title of Duke and Bequeathed Lord Soul Shard out of gratitude for his aid in the war.

Quest with Immortality
Seath eventually went insane in researching the Scales of Immortality as he could never have them for himself. However, he no longer requires the Scales as his research into the Primordial Crystal (which he stole from the dragons) resulted in him becoming an Undead, and thus, immortal. He has his servants abduct women for him to perform experiments on and is able to use sorcery to create life from nothing. It is suggested that he is the father of Crossbreed Priscilla.

Imprisoning Chosen Undead
When the Chosen Undead reaches the Duke's Archive in search of Seath's Bequeathed Lord Soul Shard to fill the Lordvessel, the albino dragon sends his minions out to defeat them. Upon reaching the top of the Archive, Seath ambushes the player and traps them in his room. He regains all lost health immediately and easily overwhelms the player, thus they must allow Seath to kill them. They respawn at the bonfire in the prison section, forcing them to fight through Seath's monsters to fight against him again.

Downfall
From Big Hat Logan the player is informed of the Primordial Crystal in the nearby Crystal Cave that grants Seath his immortality. Upon reaching the room with the crystal, Seath flies over to stop them. Despite his efforts to protect his Primordial Crystal, the Chosen Undead shatters it, allowing them to damage Seath and slay the tyrannical dragon.

Personality and Traits
In contrast to his brethren, Seath the Scaleless has more centaur-esque stature, possessing slug-like lower half that has a pair of thick tentacles as sort of legs. He also lacked scales unlike typical everlasting dragons, exposing his fleshy skin. Like other everlasting body, his upper torso and arms almost humanoid in shape. While seemingly blind due to lack of eyes, his sorcery enables him to perceive his surroundings in the similar manner that eyes do.

Envious and bitter by the fact that he was the most different and defect of his kind, Seath chose to betray his fellow dragons by joining the Lords, particularly Gwyn, upon discovering that their powers can shatter Everlasting Dragons which render them mortal. Arrogant and power hungry, he believed that he could gain immortality for himself and seemingly success with Primordial Crystal. His obsession and immortality not only resulting the loss of the innocence of women that he mainly turned into monsters, but also drove him insane than he let on. This and the reward of his betrayal, however, proved to be his downfall.

Trivia

 * The name Seath is from the dragon with the same name in King's Field, he's the rival of another dragon named Guyra (who Black Dragon Kalameet is based on).
 * He is also connected with the Moonlight Sword, a symbolic recurring weapon from King's Field.
 * Despite being blind, Seath can still detect the player even with the Hush sorcery and/or the Slumbering Dragoncent Ring.