Summon Darkness

Summon darkness was a dark side Force power conjured through Sith Magic, that created visible, palpable waves of literal darkness, accompanied by a loud roaring sound. Performed by an individual adept in Sith magic, the wielder could summon the power of the dark side, using it to visibly immerse the sky and atmosphere with total darkness. This dark power manifested itself in the form of a storm cloud, complete with rolling claps of thunder. As the darkness spread, anyone in the vicinity would soon suffer from feelings of intense pain. Prolonged exposure tormented individuals with strong feelings of despair and hopelessness. Jedi and other users of the light would begin to feel smothered, as though their connection to the Force was being snuffed out, though more powerful users of the light side could withstand the darkness' debilitating effects.

This technique was believed to be lost until it was rediscovered by Darth Sirena.

Information courtesy of Wookiepedia