The Channelers are agents of Seath, who venture out into Lordran for unknown reasons.
They scour Lordan searching for Undead that they can kidnap women to take back to the Archives where they will be experimented on:
Robe Of The Channelers
Robe of the Channelers, sorcerers that serve Seath the Scaleless.
Even after the onset of Seath's madness, the "snatchers" as they were often called,
ventured to far lands to find suitable human specimens.
Seath is said to be blind, and it is implied by the Six Eyed Helm Of The Channelers description that he sees through his Channelers. In this way his influence stretches across Lordran and other lands:
Helm of the Channelers, sorcerers that serve Seath the Scaleless.
The six eyes arranged in two vertical columns compensate for Seath's lack of sight.
The Vinheim Dragon School is likely founded upon the following or worshipping of Seath and his sorceries.
They can be found in the Undead Parish, The Depths, and The Duke's Archives.