Invidia is obtained at the Entrance Area as the player's starter weapon. It's a lightning bolt weapon that looks similar to the Nemesis (of Cave Story).
According to it's description, it's mistaken for the Nemesis. The Nemesis is a powerful weapon in Cave Story that grows weaker as it's leveled up (firing rubber ducks at Level 3).
Losing this weapon causes the Crescent Moon weapon to change.