The Dark Seed are secretive cadre of Black Mandragora assassins, infamous for killing many magic-users during the Battle for the Shadows. The members of this group appear to have a number of abilities far beyond their counterparts - they can disappear into the shadows to reappear elsewhere, cloud an area with darkness, and teleport. Some appear to have returned from the dead - although there are many scholars who scoff at such ideas.
The abilities they display remind many of the Shadar-kai, who appear to hate the Dark Seed with an intensity bordering on obsession. A prospective Dark Seed member must go through some sort of initiation, and it involves the hunting down several Shadar-kai in the Shadowfell. They are then killed in some obscene rite that transfers the shadow manipulation powers to the Dark Seed member.
It's unclear as to whether the Dark Seed are a society or cult, or even their affiliation with a Noble House. What is clear is that even the Mandragora hold the Dark Seed in fear. They are killers and are utterly ruthless. One almost tipped the Battle of Koshrak's View in the Mandragora favor when he sabotaged the gates. Only the quick thinking of a group of adventurers prevented a further catastrophe.