The Void is the power to cause ultimate darkness in the room.
The ecto-entity can use the ectoplasmic energy in reverse to create the absence of light, pull all light into himself almost like a ghostly black hole.
Danny Phantom is in battle with a bunch of ghosts or a bunch of humans and they'd be surrounding him and Danny could concentrate and then pull all light into himself and cause ultimate darkness to fill the room.
The episode in which The Void is introduced would be focused mostly (if not solely) on that power. The following is a Hartman's description of what such an episode would be like: On a bright, sunny day Danny has a negative reaction to the sun (e.g. needs to put on sunglasses; doesn't want to get sunburned). At one point he exclaims "Too much sun" or "Too much light" or something else along those lines, and then suddenly everything goes dark. At first Danny doesn't realize that he caused all light to go away, but even when he realizes that he's the reason everything is dark he doesn't know how to control it and darkness follows him everywhere he goes. Due to the nature of this power, Danny fears that he's turning evil. At some point, probably before Danny learns how to control The Void, Vlad would turn this power against him and traps Danny in the very darkness he's creating and Danny would need to survive in that darkness.
- Wouldn't be able to do it for a long time. It takes intense concentration.
- Uses reluctantly. The main reason he uses it sparingly is because the ability may be trying to take him over. Hartman made the analogy to Venom in the Marvel universe. However, the reason for this power's drive to the dark side is unknown.