What is Velka? She is a goddess, more specifically, goddess of sin, not the commitment of sin, the punishing and pardoning of all sins (commited by anyone, god or mortal) now i'd take Velka is not the kind of crazy religious punisher from hell, she seems kind enough to pardon all kinds of crime... for a price, and hell what a salty price (except heinously horrible ones, such as murdering another god)
She was part of Gwyn's pantheon of known gods (sharing the table with Flann, Gwynevere, Fina, the Blacksmith Deity and the Firstborn, Nito was not part of the Pantheon but its fair to assume he is a god of death) and it's very fair to assume they feared and respected her, mostly over her vast power, since all sin is her domain (see Oswald dialogue) mostly the gods felt a bit uneasy around her
After the incident with the demons, the linking of the flame and the firstborn taking over his father domain, it is unknown of her actions during this period, and since there is no concrete and solid in-game evidence that the Dark Ember is her creation, aside from it's location, we can't say it was created by her (i'll make a post on TPWA later on and clear this out)
So then hell broke loose and the Firstborn was stripped of his powers and removed from the annals (either betrayed, or by doing something stupid doesn't matter) and we are left with very few reasons for that to happen,(SPECULATION PART, NOT BACKED BY SOLID EVIDENCE, ONLY BY CONVENIENCE AND PUTTING 2 AND 2 TOGETHER) for me it was a plot mastered by Gwyndolin, who, jealous of his brother praise by his father, crafted a way to cast him down and be the ruler himself (hence the link the fire ending) but which reason he used to convince the others in the Pantheon to go with him? I say the Firstborn was not happy with the harvesting of the humans, and thus tried to change that, and Gwynny used that to frame him as a betrayer, and thus, we know what happen
And where does Velka fit in that? let me ask, don't you guys think that halting the course of nature by crazy means and instructing his peers to harvest the humanity of the humans (see Kaathe dialogue) to keep the fire up, prolonging the power of his people for another thousand year, never allowing nature to run it's flow is a sin? because for me it is, and a hell of a big one, and Velka was not happy at all by this, even though she was a goddess herself, she would never fade, since she was needed by humans, and even if all other gods were cast down, she would still be needed, to punish the wicked, so she too sided with the humans along with the Firstborn, and before his fate could befall her, she fled, to somewhere unknown (maybe Carim? Would explain many of her followers being from there, and maybe she was not alone? maybe Fina tagged along, but i digress)
something i found out to be really interesting about her, snugly, the invisible merchant, will exchange a sunlight medal for a white titanite chunk, which is used to reinforce Occult weaponry, so if Occult is really a creation of Velka, to punish gods, then this a great link between her and the Firstborn
so to finish, i say the huge crow that snatches the CU and brings him to Lordran is either her herself or one of her servants, and that she spread the Chosen Undead thing to try and find a way to set nature back into it's course and let the age of Dark be Unleashed (even though her plan can backfire, with the CU choosing to maintain the age of fire)
I won't go deeper into other speculations, like Priscilla being her probable daughter or not, since i did not delved enough into the Priscilla matter, mainly this is all i got, i hope you enjoy, and Shkar, stay away from my posts...