Disables your opponent from using all quick-play spell cards, which are commonly used in many decks.
High level monster that requires 2 tributes, or some investment to special summon.
Disabling your opponent's quick-play will mean that your opponents can only activate traps when you are attacking, or sphere kuriboh. Bring cards like Trap Jammer, Seven Tools of the Bandit, or Overwhelm to make sure that your attack will go through.
Commonly used in many decks due to switch position of your monster(s), or take control of them, using Invader of Darkness will prevent your opponent from using this altogether. Your attacks can only be stopped by traps or Sphere Kuriboh.
Summoning a normal monster together with this spell usually means safety ensured, no matter how low their attack is. Summoning Invader of Darkness will definitely shut down a normal monster deck, just be aware and not attack carelessly since most people still use Mirror Wall.
Used to lower a monster's attack, or make sure they survive, this card can be activated in both players' turn and also on the damage step. Using Invader of Darkness will prevent your opponent from being able to use this.
Prevents activation of your opponent's Spell Cards
Since virtually every deck runs econ and harpies is likely to run otc, I think this is a pretty strong card. It runs well with jowls of dark demise and feint plans to steal your opponent's resources and tribute out invader. Run it with the octhros guy and necrofear and you have a lot of synergy right there.