(covers information from several alternate timelines)
The Sphere-Builders were a trans-dimensional species and a faction in the Temporal Cold War. Possessing advanced temporal technology, they were responsible for the creation of the Delphic Expanse spheres in an effort to reconfigure space to be habitable for their species, as a prelude to invasion.
The Xindi referred to the Sphere-Builders as "the Guardians," and revered them as religious figures. After the destruction of the Xindi homeworld, the Builders repeatedly appeared to the Xindi survivors, guiding them to habitable planets and valuable resources. Similarly, the Triannons referred to them as "the Makers," and believed that they were transforming the Delphic Expanse into a paradise for their eventual return. (ENT: "Chosen Realm", "The Council")
Using their ability to examine alternate timelines, the Sphere-Builders discovered that, in the 26th century, the forces of the United Federation of Planets would eventually repel their invasion, and drive them back into their trans-dimensional realm. In an attempt to prevent the formation of the Federation, the Builders contacted the Xindi and falsely informed them that Humanity would eventually be responsible for the destruction of their race, and that the only way to prevent that would be to destroy Humanity first. They assisted the Xindi in the construction of a prototype weapon, supplying them with technology from the 26th century, and the weapon was subsequently launched against Earth, resulting in the deaths of seven million people. Wishing to oppose the Sphere-Builders, a representative of another faction in the Temporal Cold War informed Captain Jonathan Archer that the Xindi were constructing a far more powerful weapon, designed to destroy all of Humanity. Captain Archer subsequently took Enterprise NX-01 into the Delphic Expanse to find the Xindi. (ENT: "The Expanse", "Azati Prime")
The Sphere-Builders also assisted the Xindi-Reptilians in their attempt to synthesize a bio-weapon with which to destroy Humanity, a plan which had been disapproved by the Xindi Council. In order to help them keep their efforts a secret from the rest of the Council, the Builders transported three Reptilians back in time to Earth, the year 2004, where the Reptilians worked on the virus. Captain Archer and T'Pol were eventually successful in stopping the Reptilians, having been transported back in time by Daniels, a temporal agent from the 31st century. (ENT: "Carpenter Street")
Enterprise first encountered the Sphere-Builders when they discovered a test subject in a small pod inside a trans-dimensional disturbance. As soon as the test subject was removed from the disturbance, he began to suffer from a form of rapid cellular degeneration, a result of existing outside of his trans-dimensional realm. Before he died, he attempted to destroy Enterprise to stop its crew from completing their mission in the Expanse. (ENT: "Harbinger")
When the Xindi were confronted with evidence provided to Archer by Daniels that the Sphere-Builders were manipulating them, the Builders began to lose their credibility with a few of the Xindi species. This forced the Builders to confide in the Reptilians and Xindi-Insectoids, using the Reptilians' and Insectoids' hunger for power to their advantage. Attempting to ensure their victory, the Builders promised the Reptilians that they would rule as the dominant species of a new Xindi empire, if they hijacked the weapon and attacked Earth. As Captain Archer took a team to stop the weapon, Enterprise attempted to disable Sphere 41. The Builders boarded Enterprise and attempted to destroy the ship, but were unsuccessful in stopping the crew from disabling an interspatial manifold on the surface of the Sphere using a deflector pulse. This resulted in a chain reaction which destroyed the entire Sphere network and caused the Builders' trans-dimensional space to collapse. As the Delphic Expanse returned to normal space, becoming incompatible for them, the Builders were forced to retreat back into their native realm. (ENT: "Countdown", "Zero Hour")