X was the first highly powerful individual from a multi-dimensional godlike species known as the X who intentionally changed his species from Q and left the Q Continuum. (Star Trek: Phoenix-X)

History

At some point, X was a Q and changed his name and species to X, prompting an upset Q Continuum to imprison him in an omni sphere, inside an anomalous nebula.

In 2376, the USS Phoenix-X retrieved the sphere from the nebula and opened it, releasing X and summoning Q. Not wanting to go back, X sacrificed his life and transferred all his power into Captain Daniel. ("The X Continuum")

Legacy and X power

The sacrifice of X inspired more Q from the Q Continuum to change their names to X and begin to lay a groundwork toward a future X Continuum. Their first step was to reduce Daniel's given X power and use it to create X Time. Other Xs shared their power among select chosen mortals throughout the Milky Way Galaxy so they would grow it amongst themselves. These mortals and Daniel would be known as Omni. ("Vengeance")

Later, Q and Klokian remedied that when Klokian led Q to the X so that Q could force all the X to return to the Q Continuum. With that, balance would be restored. ("Avalon Battlefield, Part II")

During this forcing by Q, the base X power that was within Daniel and the other Omni was returned to the Q Continuum as well. But, by that time, Daniel and the other Omni had gained enough experience and growth to retain their own X power. ("Avalon Battlefield, Part II")