The Eternity Crystal was allegedly the pinnacle of Jerni science, capable of winding back time from a horrendous event, and guiding the future (present) in a different and more positive direction.
The crystal was supposedly hidden in the Jerni ruins near Adony Station, but it turned out the whole thing was a hoax devised by Darth Vader in an attempt to kill Luke Skywalker and Leia Organa.
- The Complete Star Wars Encyclopedia, Vol. II, III