You can activate “Enemy Controller”’s first effect even if you have no monster on your side of the field.
If you activate either of “Enemy Controller”’s effects, and your opponent chains “Book of Moon” to flip the target monster face-down, its targeting conditions are no longer correct when “Enemy Controller” resolves and “Enemy Controller”’s effect disappears.
The monster you Tribute for “Enemy Controller” is a cost.
When the Ignition Effect of “Cannon Soldier” or “Chaos Emperor Dragon - Envoy of the End” is activated, and you chain “Enemy Controller” to take control of the “Cannon Soldier”/”Chaos Emperor Dragon - Envoy of the End”, even if you take control of the monster, you cannot take control of the effect itself. So if your opponent activates “Cannon Soldier” or “Chaos Emperor Dragon - Envoy of the End”’s effect, and you chain “Enemy Controller”, you still take the damage from “Cannon Soldier”/”Chaos Emperor Dragon - Envoy of the End”.
If “Dark Magician of Chaos” is Summoned, and you chain “Enemy Controller” to the activation of his effect, even if you take control of “Dark Magician of Chaos”, you cannot take control of his effect, so the Summoner (the activator of the effect) gets to retrieve 1 Spell Card from the Graveyard.
When “Spirit of the Pharaoh” is Special Summoned, and “Enemy Controller” is chained to the effect, the player who Special Summoned “Spirit of the Pharaoh” gets the effect because he activated the effect.
If you take control of your opponent’s monster with “Enemy Controller”, and then the monster is flipped face-down with “Book of Moon”, control of the monster is still returned to the opponent (the original controller) during the End Phase.
If you take control of your opponent’s monster with “Enemy Controller”, and then the monster is removed from play with “Interdimensional Matter Transporter” or “Dimensionhole”, when it returns to play it comes to the side of the field of the player who used “Enemy Controller”, but then immediately moves back to the original Controller.
If your opponent Normal Summons “Breaker the Magical Warrior”, “Breaker the Magical Warrior”’s effect to place a Spell Counter on him activates, and you chain “Enemy Controller” to that effect and take control of “Breaker the Magical Warrior”. In that case, resolve “Enemy Controller” first and “Breaker the Magical Warrior”’s effect of placing a Spell Counter second.