Word to describe someone who secretly creates trouble?
The Word agitator is possibly what you are thinking of. Rabble-Rouser is a common expression but they tend to be be used for both non visible organisers and active participants who stir up the crowd during demonstrations.
Drawing on the empowering imagery of motherhood, a few women attained prominence as agitators and publicists and many more participated in public demonstrations.
agitator Cambridge English Dictionary noun: disapproving someone who tries to make people take part in protests and political activities, especially ones that cause trouble:
rabble-rouserCambridge English Dictionary noun; a person who says, writes, or does things that make crowds of people excited or angry, usually intentionally in order to make them act in a particular way:
Johnson was unpopular with the management because he was a well-known rabble-rouser