The One is a mythic creature, the God-to-the-Gods. The Gods believe that the One is the architect of all reality.
The Sandstriders first heard about The One when the Council of Three mentioned it in Vuul'kira. The issue came up again later, when they discovered a slab of wall featuring a likeness of Laz Ursulus that ostensibly originated in the Temple of the One. Most of Episode IV: Astral Rifts was spent looking for the temple itself, which the Sandstriders eventually found.