Grey Gentlemen

The Men in Grey, also known as the Grey Gentlemen, are the antagonists of the German novel Momo by Michael Ende and it's cartoon series and it's live action movie with the same name. They're some kind of supernatural organization which members are supernatural men with grey clothes and grey skin who plan to take over the world by stealing the time from the humans.

Powers/Abilities
The Men in Grey are completely immortal, the only way to kill them is to keep them from smoking their cigars, because they would die without them. The cigars are made of rotten hour-lilies. Hour-lilies are represantations of the time of a human. As much time he loses, as much the flower gets rotten. The Men in Grey try to convince people to save time to make their flowers rotten and create more Men in Grey to replace the human race. When they leave a person they talked to, the person forgets about their existence.

Novel
In the novel, they're trying to steal time by manipulate people with stupid promises or deals. They try to make several deals with the main protagonist, the girl Momo, to keep her from stopping them, but they all fail. Finally, Momo closes the door where the last six Men in Grey who are alive at the end keep the stolen time-lilies. Five of the six last Men in Grey die by losing their cigars, the sixth one gets the time-lilie Momo's got, but feels heart-warmed and dies suddenly, saying it's good that it would finally be over.