What Velka does - the curse on humanity
In this text I will explain what the will of Velka is. Instead of going off deep speculation, I think 4 setpieces of the games and 1 lore description paints her story.
So first, it is obvious that she punishes sinners, but she also provides absolution. In exchange of souls, any sin is forgivable. Oswald of Carim indeed represents a pardoner, but everything about it speaks corrupt.
This line of thinking reveals her actions. Why? Because of the most mysterious item of hers, ring of sacrifice:
"This mystical ring was created in a sacrificial rite of Velka, the Goddess of Sin.
The magenta-shaded ring is especially rare. Its wearer will lose nothing upon death,
and will be freed from any curse whatsoever, but the ring itself breaks"
First, the magenta shows that this relates to Velka based on her description. But what is less obvious is that this expands on the less effective ring. Mechanically it prevents curses, but the original, green ring, prevents death the same.
You will lose nothing on death! She sold this premise to the humans, as a mockery and revenge to Gwyn's curse.
The painting confirms how she achieved this. And yes I mean the one in Friede's room. There are 2, and this is important, two witches of izalith, given away by their tattered clothes, with a sword that looks related to Carim or the Sable Church.
They have stabbed two white snakes, representing covetous ring: covetous souls!
They are also burning humans. They are in dark woods, a common and plausible area, close to a church. This could represent the church of Londor.
There are 3 sable sisters, and I think this is also important. As the ring of sacrifice, prevents curses, but has three people. So I presume what is happening here reflects on Friede.
She left her sisters and gave up on choosing fire. Meaning she gave up on sacrificing humans.
It should be clear that the witches of Izalith are the ancestors of the 3 sisters, and they have carried on their will!
Yet, their will seems to be burning and killing humans and greedy serpents? There is an answer to this too: The new londo and ringed city soul masses, look like they are a burnt, melted mass of souls.
CONCLUSION:
Velka gave humans a mean to fight the gods via sacrifice. The implication is that she is a witch of Izalith, behind the disasters of flame in the games. I think that her soul could not die due to her powers, but then it transformed into the soul of Rosaria over time. Because there wasn't humans to sacrifice. Her sacrificial rite involved burning someone who had a dark soul. This likely caused the Izalith incident (possibly ending her life, at least for an age).
