OP, to cut a long story short, the Illuminati is a supposed cult that apparantly exists in the realm of showbiz, sending out subliminal messages to the public. Basically it's the devil trying to control the public via the world's favourite celebrities. These celebrites are usually people desperate for a break in music and superstardom, so they sell their soul to the devil, who makes them really famous, meaning they earn a shitload of money and success. But, the only catch is, they are the devil's forever, and the only way out is death. Musicians said to be a part of this cult are Jay-Z, Beyonce, Rihanna, Lady Gaga, Michael Jackson, The Killers, Kanye West, 30 Seconds To Mars, T.i. and Alicia Keys, to name a few.
It's all bullshit though. But it's certainly interesting, and entertaining how deluded, paranoid and gullible some people can be. They are just honest musicians trying to make a living out of their talent.
One of the interesting Rihanna theories that I have read about though is the one over the Chris Brown incident. Apparantly he only beat her to get the devil out of her, because he wanted the old Rihanna back, and was so anti-illuminati, hence why he was never really accepted in with the whole Jay-Z circle. I personally think it's because he was young and stupid and couldn't control his temper because he is a stupid prick