A practitioner of the Jar'Kai technique, Andru wielded two purple-bladed lightsabers in combat. During his life on Dromund Kaas, Andru sought after ancient writings describing a powerful dark side ritual which would give the user mastery over the Force-walking skill.
Long after his death, Darth Andru's spirit was locked within the halls of the Dark Temple where it seethed with an undying hatred. During the height of the Cold War, Andru's wraith was confronted and subjugated by the Sith Inquisitor known as Kallig who was able to master the Force-walking technique and consume Andru's spirit. Absorbing Andru's energy and adding that to Ergast's, Kallig confronted Dark Councilor Darth Thanaton and was soundly defeated, even with the added strength.
However, the bond with the two ghosts kept Kallig from dying. The Sith Lord then decided to find more ghosts and use their power to defeat Thanaton. After adding Kalatosh Zavros and Horak-mul to the collection, Kallig returned to Dromund Kaas to face Thanaton.
However, upon drawing on the power of all four ghosts, it became too much for Kallig to handle and was stricken with a sickness. The power would deteriorate Kallig's body and the ghosts threatened to drive them insane.
Revan and Malak's Sith Empire