Death Eraser

The Death Eraser is a plot device exclusive to the Death Note pilot manga.

Overview
The Death Eraser is a special artifact that can erase the names in the Death Note and revive the people that have been killed with the note. However, it only appears in the pilot manga: feeling remorse after killing with the Death Note, Taro Kagami is offered the Death Eraser by Ryuk, and is told that he can erase the names in the Death Note and the victims will miraculously come back to life. He does so, and discovers the following day that the bullies that he had killed with the Death Note are revived...

In the Anime and Manga
Since its existence is only true in the pilot manga and does not exist in any other Death Note media, in the main manga and anime it is revealed that if a cause of death or the details of death are struck out with two straight lines before the forty seconds has elapsed, then they would be cancelled and can be changed if desired; but, no matter what, once the name of a victim is written down in the Death Note, he/she will die (Chapter 107).