Cult of Orcus
Originally masquerading as the Cult of Bane, they set up cells in all the main cities in Nentir, as well as Scarvale. The Scarvale group was wiped out by the Cave Killers, although their leader escaped (later captured by Lord Protector the druid, and ripped apart by a treant during interrogation).
They wear red and black robes, and are obsessed with raising zombies to serve them. They even have ninja zombies!
A fortune teller saw a vision of men in red and black robes holding the necklace that Echo was trying to find. It belonged to his late wife and was stolen from them by goblins.
The cult leader who was killed by a treant was posthumously interrogated again by Horkin the mage, using a “speak with dead” spell. He indicated there was a larger presence of the group in Fallcrest, and a leader there in charge of him. He wouldn’t give up the name, but Horkin found it by other means. He did not tell the group the name, he wants independent verification before he does anything.