A hacker managed to hack into the Battle.net server, which had been considered unhackable and non-cheatable until then, in the midst of all the Christmas rush. The stranger managed to overcome the security system through a security gap in the authentication.
After that, objects were stolen and some characters were simply deleted. In this way some top games have been robbed of their heroes. This has been happening since December 19th, but it wasn't noticed until December 31st. Blizzard now wants to revive all hardcore characters who died during this time and give all those stolen the opportunity to return to the status of the 19th.