The Nightmare Mode is A Evil Version of The Omnitrix. It Can Only Be Activated By Zombozo.
Different Types Of Alien's
The Nightmare Mode Makes Ben Alien's Look Evil, Like Horns, Vampire Wings and More.
In some series, it also gives them different powers and causes them to be hard to control.
Zombozo secretly installed this function on the Omnitrix. He used a radio signal that would also affect any Omnitrix made after that, meaning it was on every Omnitrix Ben ever had.