The One was the main antagonist in Star Trek V: The Final Frontier, this entity claimed to be the supreme being God but was actually a malevolent noncorporeal being.
The One claimed to be the source of monotheism on a number of planets across the galaxy. It was an immensely powerful alien entity. Approximately one million years ago after Q brought the being 0 into the universe through the Guardian of Forever. With Q's help 0 brought The One into the universe as well along with the Beta XII-A entity (aka *) and Gorgon. The young Q thought the group was going to have some fun but he was totally unaware of how evil they all were.
After the group wrecked chaos across the galaxy including destroying the ancient Tkon Empire the rest of the Q Continuum attempted to arrest them. After a prolonged battle across time and space four Q working together were able to destroy The One's body by sheer will leaving only the head. For The One's many crimes he was imprisoned at the center of the galaxy until he either repented or the heat death of the universe - whichever came first. The Q built the Great Barrier around a star system at the center of the galaxy and imprisoned The One there. 0 meanwhile was banished to the space between galaxies, and the Galactic Barrier was built around the galaxy to keep him out of the galaxy.
Some Vulcans learned of The One and the planet - which they called Sha Ka Ree - at the center of the galaxy beyond the Great Barrier where The One was held. In the 23rd century the Vulcan Master T'Rea taught her son Sybok about God and Sha Ka Ree. After her death Sybok went to live with his father Sarek and his wife Amanda along with his half-brother Spock. When Sybok broke in to the Hall of Ancient Thought to meld with his mother's katra he was banished from Vulcan. In the years that followed Sybok continued his studies. Using brainwashing Sybok was able to amass a large number of followers and commandeered the USS Enterprise-A in 2287 to go to Sha Ka Ree. Penetrating the barrier the Enterprise crew discovered the planet that Sybok believed to be Sha Ka Ree.
Sybok along with Kirk, Spock, and McCoy transported down to the planet. The Entity on the planet claimed to be the idea of God found on Earth and a number of alien worlds. He demanded the Enterprise be brought closer so he could escape. The others were lost in amazement over what they were seeing, however Kirk maintained the presence of mind to wonder why God would need a starship. When Kirk asked God why he needed a starship the entity struck him with bolts from his eyes, to the horror of the Enterprise crew and the others on the surface. The One then attacked Spock when Spock also asked what God needed with a starship. McCoy was next in line when he said that he'd doubt any God who inflicted pain for its own pleasure, however Sybok intervened.
When Sybok said that the God of Sha Ka Ree would not do this The One responded that Sha Ka Ree was a vision Sybok created. He then revealed to Sybok that he was not the benevolent God Sybok was searching for. Sybok wrestled with The One to buy Kirk and the others time to escape. The Enterprise fired a torpedo which disabled The One for a few moments, giving Kirk, Spock, and McCoy time to run back to the shuttle. The One pursued Kirk, Spock, and McCoy to kill them. Kirk had Scott beam Spock and McCoy back to the Enterprise. The One chased Kirk up a cliff where he was about to kill Kirk. However Kirk was saved when Spock used a the weapons of a Klingon Bird of Prey against The One, injuring the being.
After 0 was able to return to the galaxy in 2374 he was able to briefly take control of the Enterprise-E. 0 stated that he had prisoners to free, meaning The One. Fortunately Q and the Calamarain put their differences aside and worked together to defeat 0 long before that happened, banishing him from the galaxy once again.