The Slavers are an unknown team of villains in The Edge Chronicles. Everything about them is unknown - their species, their gender, their location. But is is presumed, from hints in the books, they could have been Goblins from Undertown.
The Slavers killed Rook Barkwater's young parents and left him mercilessly to die in the dark Deepwoods. The Slavers believed him dead, but he was saved by a banderbear.
The Slavers are responsible for Rook being an orphan, his constant nightmares, his fear of woodwolves, and his freedom fighting. The Slavers are also indirectly responsible for almost everything that happens in the trilogy Rook appears in - for instance, his living with the Librarians, his journey to the Free Glades, his meeting with Xanth Filatine and Vox Verlix, and his ultimate discovery of the maelstrom which would destroy Undertown.
The Slavers were presumably successful dealers; or ruthless barbarians.