The Plot is largely borrowed from a Link to the Past, Agahnim is a wizard whom Ganon possesses to conquer the world of Light, there's also a dark realm which is the inspiration for or directly copied from in TP's twilight.  And Agahnim is trying to force open a link between the dark (originally called the Golden realm before Ganon changed it) world and light.

  Basically, the reason I feel like I played TP ten years ago, is because I did.