The dark lord Sauron had been searching for this ring
for a very long time by this point - millenia, really,
although millenia might indicate otherwise. Now, this
One Ring was almost, but not quite, completely unlike
any other ring. Which wasn't to say it wasn't a ring
at all, because it was - what made it unlike other rings
was the fact that it had a mind of its own. This caused
a great deal of trouble amongst other reputatable ring-makers
of the time who, although saying otherwise, did not
really put blood and sweat into their own rings; consequently,
they gathered together to destroy the dark lord, which
very much didn't please him.
Since the dark lord Sauron wanted his ring back, a fellowship
decided to keep it away from him, despite all their
knowing consisted of the idea that "very bad things
would happen." Despite very bad things already happening
(the dark lord was quite unhappy about being mostly-destroyed
some aeons past), they felt obligated to take the One
Ring and toss it into a boiling volcano just beneath
the dark lord's eye - which, of course, was a very silly
thing to do.