Black Hand

The Black Hand was what Gollum referred to as the hand of Sauron which had once bore the One Ring. It also represented Sauron's evil influence just as it did his fallen master Morgoth.

History
Unable to assume a fair form, Sauron's hand was blackened after the death of his body in the Fall of Númenor. When Isildur cut his ring-finger off with the shards of his father's sword, Sauron's one hand had four fingers.