Egeria was one of the Camenae, the Latin goddesses of springs and streams, and wife (?) -- Juvenal mockingly calls her amica -- of Numa.

Tradition had it that Numa consulted nightly on the Caelian hill with Egeria, in a grove which was watered by a spring flowing from a nearby cave. There she instructed him to establish the civic and religious rites for which his reign is well remembered.

Piranesi: Egeria's Grotto
A sketch by Piranesi of the 18th century remains of Egeria's sacred grove

Livy recounts the legend as a subterfuge created by Numa which enabled him to claim divine inspiration for the civic and religious practices he instituted, and thus turn the Romans from thoughts of war to peace (see Ab Urbe Condita 1.18 and 19).