Transdimensional beings who built the spheres of the Delphic Expanse and manipulated the Xindi into attacking Earth to prevent a future defeat.
The Sphere Builders are beings from a realm outside normal space who cannot survive in this universe for long without special conditions. They built a network of huge spheres throughout the Delphic Expanse that slowly altered space there, distorting physics and creating dangerous anomalies, with the aim of making the region habitable for themselves.
The Xindi revered them as the Guardians and followed their guidance. The Sphere Builders could see possible futures, and they knew that in the 26th century the Federation would defeat them. To prevent this, they persuaded the Xindi that humans would destroy their world, which led to the attack on Earth in 2153 and to Enterprise's mission into the Expanse.
In the third season finale, Zero Hour, Jonathan Archer stops the Xindi weapon while T'Pol and Trip Tucker lead Enterprise in destroying Sphere 41. The loss of that sphere collapses the whole network and ends the Sphere Builders' plan. Their story turns the Xindi arc from a war of revenge into a plot about manipulated fear.