False Light Side Aura

False light side aura was a dark side Force power conjured through Sith Magic, that covered a darksider's dark side aura with one of light. Dark Lord of the Sith Darth Zannah used this to infiltrate the Jedi Archives on in the Jedi Temple on Coruscant. Using this spell she fooled even Jedi Masters into thinking she was a follower of the light.

Information courtesy of Wookiepedia

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