We present the first optical validation of an X-ray line-emitting object (XLEO) discovered in the XMM-Newton observation of the Chandra Deep Field - South. The object is an active galactic nucleus at z(x) similar to 0.66. An optical spectrum of the source indeed confirms and refines (z(o) = 0.665) the redshift obtained from the X- ray line. The X-ray and optical properties of the source are presented and discussed.