An immortal {demon, god, sage, whatever} who roams the world offering a pact: whoever shall accept their contract will receive all of the knowledge, all the memories, all the secrets, all the thoughts of everyone who has taken this deal before them. The catch is, of course, that all of your knowledge, memories, secrets and thoughts are added to the pool.
Politicians risking state secrets for the chance to get power. Puppets gaining the upper hand over those who asked them to take the pact in theur stead. Old mysteries best left unknown. Common people learning more than they should.
Knowledge is power, but after reaching the top step it's only a matter of time until someone steps on you.