The Ephors began as a branch of the Spartan government during the period of the Lycurgian reforms (circa 650 B.C.), after previously acting as private advisors to the kings. The office lasted until 227 B.C. when King Kleomenes III abolished it.
The Ephors main political adversary was the kings, since they appear to have acquired power from the Assembly and Gerousia early on without much difficulty. But the kings fought against their growing political power and the strongest ones succeeded in containing them.
Kings had absolute power when on a campaign and the Ephors had no jurisdiction outside of Sparta, although a pair of them usually accompanied the king on his campaigns. They had no authority to interfere with the campaign or any political activities connected to it, but were known to gather information that could be used to prosecute the king when the conflict was over.