The One

The One was a malevolent entity from another dimension. Approximately one million years ago, 0, with Q's assistance, used the Guardian of Forever to bring The One, Gorgan, and (*) into the Universe. The group committed many atrocities, including the destruction of the T'Kon Empire.

The Q Continuum eventually banished The One behind the Great Barrier, where he remained for half a million years. (TNG novel: Q-Strike)

In 2287, The One manipulated Sybok into commandeering the USS Enterprise-A and taking it through the Great Barrier in an attempt to gain freedoom, but was defeated by James. T. Kirk. (TOS movie: Star Trek V: The Final Frontier)