Dissonance is the anti-Resonance force of the Matrix. It is possible that Dissonance and Resonance are actually the same thing. Others believe it to merely be a warped personal interpretation of the Resonance. And still others think it may be a creation of an unknown AI.
Dissonance technomancers and otaku often believe that data should be corrupted. They thrive on instability and glitches within the Matrix and believe this is the way the Matrix should be. Others are technological supremacist, who believe that non-resonance users are lower lifeforms or that believe that AIs should be revered as gods.
The first known follower of the Dissonance was the otaku, Pax. After Deus abandoned her, she escaped the SCIRE and heard the callings of the Dissonance. From there she formed Ex Pacis and was able to find other like minded otaku using the Dissonance.
Despite Pax being the first known dissonance otaku, it is possible that there were dissonance otaku before her. It is said that otaku who fail to meet or understand the Deep Resonance became out of sync with the Resonance. So while it's possible that there were dissonance otaku before Pax, they were unorganized and no threat until Pax started Ex Pacis.