The Dark Remembrance
The Circle of Thirteen
You know little about this organization other than they are exactly the other half of the ritual magic circle that Cornelius was involved with.
They split up because of ideological differences, and you have to come to suspect it was a battle of good vs. evil. These ritualists cursed Cornelius with a horrible curse that afflicted him for untold years, and they were sent to kill him during the Slaver Battle in the Town of Nessit,
Although originally a circle of Thirteen, you killed three or four members who wore black robes and were identified to be members of this organization.
The image above is the same image from the vision given to you by Heimdall when you broke the Curse on Cornelius.
From what you have seen these ritualists are all Wizards of considerable power as they are able to conjure webs, hurl fireballs and fly (although they were using technology to do so)