Who is a small, slimy, dark creature?From 'The Hobbit' and 'Lord of the Rings.'

You've got to be  referring to Gollum, the first keeper of the ring. Possession of the ring over time warped both his mind and body, and his one obsession is to get it back. He finally does manage to wrest the ring from Frodo's hand but in doing so loses his balance and plunges headlong, along with the ring, into the firey pit of Mordor. The way the story is presented suggests that this was Gollum's predestined fate and role - to  bring about, though unwillingly, the destruction of the infamous ring.