The Dimensioneers were a powerful race of interdimensional travellers who sought to cross the dimensional divide and bestrode the infinity of dimensions. They predated the Time Lords and were predecessors to them, with the Reborn Master speculating that they even influenced the Time Lords' rise in some fashion.
They created interdimensional corridors with which to cross the space between dimensions. They also created the dimensional nodes, which the Master considered made the Key to Time look like a trinket. The two nodes, when linked to the node activator, required a Time Lord to be slaved to them, and could access interdimensional energy. They were eventually defeated and banished by the Time Lords, and their technology locked away as they fell into legend. (AUDIO: Dominion)