User blog comment:Aftenshnoshnikr/Witch Of Izalith, Chaos Sorcerer Supreme/@comment-5337347-20130703112713

The Witch of Izalith is the Bed of Chaos. Following her attempt to recreate the First Flame, she and her Daughters of Chaos were corrupted, becoming the forms we see in-game, such as Quelaag and Quelaag's sister.

As for the exact method used to make the attempt, it likely had something to do with the Lord Souls, which were born of the First Flame. The description of the Lord Soul obtained from the Bed of Chaos mentions an attempt to recreate the First Flame using "a soul", which leads us to assume that soul would be the same Lord Soul obtained from the Bed of Chaos.

Following speculation, pieces of humanity are fragments of the Dark Soul, which was also found in the First Flame. Because of this, humanity is intrinsically linked to life.

I am like, drunk on lack of sleep, though, and can't remember where I was going with all that. I'll look at it again in the morning, and try to form some cohesive thought.