Epsilon XI was a Starfleet communications array and subspace transceiver in operation in the 24th century, and was located near Klingon space.
During the invasion of the Borg Collective in 2381, Starfleet lost signal from Epsilon XI and assumed it destroyed by hostile Borg forces. (ST - Destiny novel: Gods of Night)
The station was severely damaged by an attack from the D'deridex-class RIS D'varian and was defended by the USS Enterprise-E. There was no loss of life in the assault, but the station was unable to continue its function as a communications beacon. (TNG novel: Takedown)