Data Drain is an ability in the .hack// series that is used by several characters, the most well-known user being Kite. Using Data Drain, Kite could rewrite the data of monsters in The World, changing them into incredibly weak versions of themselves while extracting Virus Cores and rare items. It can also be used on monsters that have been infected with Data Bugs to render them vulnurable.
The downside to Data Drain is that it exposes Kite's character data to the viruses present in every monster in The World. Continued exposure to viruses causes the game to malfunction, resulting in status ailment being inflicted upon Kite's party, in addition to lost EXP. Overuse of Data Drain can result in a System Error, which will cause an automatic Game Over.
Data Drain is also used by the Phases. When used upon players, it usually places them in comas.