A supposed secret organisation that coincidentally everyone knows about. When ignorant people refuse to believe common sense, conspiracies such as 9/11 and the JFK assassination are formed. The illuminati are (allegedly) controlling everything in the world and are responsible for the death of anyone famous such as Biggie, Bob Marley, Michael Jackon and the list goes on for eternity.
It's based upon a real organisation from 18th century europe. The rumours started around about the same time as the internet grew popular (late 90's) and may be the earliest evidence of trolling.
People who believe in these stories are very arrogant and refuse to listen to reasoning and evidence and hypocritically accuse anyone who doesn't agree with them of ignorance.
Person 1: Hey, did you hear the government made 9/11 happen with bombs and shit!?!?!
Person 2: Yeah and Jay-z killed Tupac!
Person 3: I heard they killed Amy Winehouse so people would forget about the norway bombings.
Person 4 (who uses common sense): You shouldn't believe everything you read off the internet, the illuminati isn't even real.