Grey Seers and the Skaven race are taken from the Warhammer Fantasy world, published by Games Workshop.
The Grey Seers are the envoys of the Horned Rat - as interprented through the Council of 13, the powerbase of Skaven rulership. As such they have no affiliation to the Skaven clans (eg. Eshin, Moulder, Pestilens and Skryre). Notable grey seers include Thanquil and Gnawdoom - other of name are Ssikitz Sshirtaz and Skleetishisk
Wielding immense personal power, both resources and magic, they are the typical leader for the armed forces encountered by the other Warhammer races. Because of their close ties to the council, they can easily "hire" skaven of the various clans.
Being wizards of the Skaven race, they can channel the power of Warpstone into their magic, making them far more powerful than their usual opponents, though much more unpredictable. Unlike other Skaven, the grey seers' fur tend to be lighter/whiter than the common dirty brown - even to the extent that one could call them albino. They also often exhibit chaos mutations in the form of curved horns growing out from the back of their heads.
Though not a large threat to the outside world due to their intrigue and plotting, they sometimes emerge from their underground lairs with excessive force.