The Death Eraser is a plot device exclusive to the manga pilot.
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.
Feeling remorse for 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 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
The existence of the Death Eraser is only true in the pilot manga and does not exist in any other Death Note media. In the main manga and in the 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, they will die.
In the film series
A similar variation of the Death Eraser has been established in the 2017 Netflix film. A rule is established that if the victim who has been written in the Death Note is scheduled to die at a specific time, the death will not occur only if the page it's written on be burned.
- The Death Eraser will not help if the deceased person has already been cremated.