Guy Fawkes (1570-1606) was a Catholic extremist who gathered with a band co-conspirators to try and blow up James I and his parliament on November 5, 1605. Fawkes was not actually the leader of the group, but because of his experience with both military and explosives, he was in charge of executing the plan. Their effort, now known as the Gunpowder Plot, was stopped before it happened when they were discovered by the authorities in the cellar of the House of Lords with barrels of gunpowder, which Fawkes was presently guarding. Fawkes, along with seven of his co-conspirators, were tried and executed.
Now when people allude to Guy Fawkes, they are calling someone a clever schemer or secret conspirator. If someone alludes to a Gunpowder Plot, they are referring to an unsuccessful attempt to remove someone from power.
Several years ago, Guy Fawkes would have been a much more obscure allusion to mention, but after the movie V For Vendetta came out (2005), the Guy Fawkes masks, featured in the movie by the character V who was also a conspirator against the government, became a popular Halloween costume.