Bed of Chaos

The Bed of Chaos (formerly known as the Witch of Izalith) is a monstrous, twisted bed of life - the source of all demons - and a boss in Dark Souls.

Appearance
When she was the Witch of Izalith, she was noticeably human-looking, wearing an ornate set of pyromancer robes. He hands display burn-scars, not surprising as a pyromancer.

As the Bed of Chaos, she takes the form of a massive tanglement of roots growing from the walls of a capital, forming into a vaguely humanoid form. On top of this figure stands a vaquely humanoid construct of fire with long arms, one with a large scycle blade. Within the roots of this figure sits a Chaos bug, which if killed, kills the Bed of Chaos.

Powers and Abilities
As the Witch of Izalith, she held great power and sway granted to her by her Lord Soul. She was very intelligent, not only having discovered and perfected pyromancy, but she was able to use it to slay the everlasting dragons and was even able to make Izalith a prosperous city.

As the Bed of Chaos, she wields the Flame of Chaos, a divine similar to the First Flame that twists those that touch it, turning them into demons.

Dark Souls
Long ago, during the Age of Ancients, the world was unformed, with no light or dark, no life or death. All that existed were the Everlasting Dragons, and the Archtrees they called their homes. At some point however, a fire came into being in the bowels of the Earth, known as the First Flame. The flame brought disparity; life and death, light and dark. At some point, humanoid beings came into existence, and drawn to the flame, four of them found great souls of lords in the fire. The Witch of Izalith found one of the Lord Soul, the Life Soul, which he used to fight off the dragons by weaving great firestorms with her daughters of chaos.

Along with his fellow lords, and a host of other gods and warriors, Gwyn made war against the Everlasting Dragons above. When one of the dragons' own, Seath the Scaleless, betrayed their secret weakness, Gwyn and his lords were able to defeat the dragons, and make the world their own, thus beginning the Age of Fire. Like Gwyn, the Witch of Izalith became a god in the new world and ruled over her homeland of Izalith.

As the fire faded, leading to the end of the Age of Fire, the Witch of Izalith began to experiment. While Gwyn set out to kindle the first flame to keep it going, the Witch of Izalith wanted to recreate the first flame and make her own. This went horribly wrong, accidentally creating the Flame of Chaos, turning her into the Bed of Chaos. The flames spread across Izalith, transforming all there into demons, with Quelana the only one to escape.

The Bed of Chaos eventually meets its end at the hands of the Chosen Undead, using its lord soul to reach the first flame.

Dark Souls II
A long time after the Bed of Chaos's death, the fire was linked and faded again many times, and the land of Lordran that Gwyn had once ruled saw the rise and fall of countless kingdoms.

The Witch's powerful soul would remain however, and he would be reborn time and time again. By the events of Dark Souls II, the Witch has been reincarnated into the form of the Lost Sinner, an imprisoned undead in Sinner's Rise.

Dark Souls III
Long since the death of the Chaos Flame, the demons that spawned from it become a dying race. Like lava cooling on the surface, the flames that sustain the demons slwly die before they all harden into stone.

Trivia

 * It is implied that the Bed of Chaos itself is simply a large shell, controlled by the Chaos Bug within meant to be the true form of the Witch.