Local Security Authority Subsystem Service (LSASS)[1] is a process in Microsoft Windows operating systems that is responsible for enforcing the security policy on the system.
It verifies users logging on to a Windows computer or server, handles password changes, and creates access tokens.
Forcible termination of lsass.exe will result in the system losing access to any account, including NT AUTHORITY, prompting a restart of the machine.
Because, lsass.exe is a crucial system file, its name is often faked by malware.
If it is running from any other location, that lsass.exe is most likely a virus, spyware, trojan or worm.