As an archetype, the archenemy is the ultimate threat to a particular individual. Their very purpose is to be the greatest antagonist to another, usually a protagonist or hero. They often represent the certain characteristics that are the opposite of their opponent: good vs. evil, light vs. dark, chaos vs. order. In many cases, the archenemy is the living motivation for the work of their opponent; defeating them is the protagonist or hero's greatest goal.
904.9. Immediately after the archenemy's precombat main phase begins during each of his or her turns, that player moves the top card of his or her scheme deck off that scheme deck and turns it face up. This is called "setting that scheme in motion." (See rule 701.22.) This turn-based action doesn't use the stack. Abilities of that scheme card that trigger "When you set this scheme in motion" trigger.
505.3. First, but only if the players are playing an Archenemy game (see rule 904), the active player is the archenemy, and its the active players precombat main phase, the active player sets the top card of his or her scheme deck in motion (see rule 701.22). This turn-based action doesnt use the stack.
3) It's meant for 3 on 1 typically, but it's up to your group. If you have 2 players with really powerful decks, you could probably make 2 on 1 work. 4 on 1 starts to get unfair for the archenemy unless you make adjustments. But like all casual variants, it's meant to be fun, so make whatever changes make it fun.
Effects that say "you" refer specifically to that effect's controller. In this case, that is the archenemy (who set the scheme in motion). Artifact spells that other players cast are not affected by this scheme.
Corrosion, the gradual destruction of materials by chemical reactions with the environment, is the archenemy of infrastructure. The deterioration of roads, bridges, traffic signals, etc., can result in unsafe conditions and lead to billions of dollars spent worldwide on emergency repairs and replacement.
The Fourth Doctor noted the Decayed Master to be his "sworn archenemy". (TV: The Deadly Assassin) The Twelfth Doctor later claimed Davros to be his archenemy much to the jealousy of Missy, the female incarnation of the Master. (TV: The Magician's Apprentice) 041b061a72