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 Tkon Empire.
In 2287, The One (impersonating God) manipulated Sybok into commandeering the USS Enterprise-A and taking it through the Great Barrier (and later, the shuttlecraft Copernicus through the atmosphere to Sha Ka Ree's surface) in an attempt to gain freedom, but was defeated by James T. Kirk. (TOS movie, novelization & comic adaptation: Star Trek V: The Final Frontier)
Though disembodied, The One had such psionic energy power over the planet that he could alter readings, speed the passage of daylight and terraform the environment. He could construct rock-like minions from the crust of the planet, or take on the substance of it for "Him"self. (TOS movie: Star Trek V: The Final Frontier)
The One was still trapped within the barrier in 2374. The malevolent entity 0 had set the USS Enterprise-E on a course to free The One, but was stopped by the combined efforts of Q and the Calamarain long before reaching the barrier. (TNG novel: Q-Strike)