You've forgotten your login password or it's locked from too many failed attempts to log...

 iCloud Account Recovery is an alternative method to unlock FileVault and reset your password if you’ve forgotten your login password or it’s locked from too many failed attempts to log in. iCloud Account Recovery allows you to use your Apple ID and password to unlock your Mac and reset your user account password, if you stored your FileVault recovery key in iCloud when you turned on FileVault. To use iCloud Account Recovery, you need to restart your Mac and enter the Password Reset Assistant from the FileVault login screen or from Recovery Mode. Then, you need to select “I forgot my password” and sign in with your Apple ID and password. Follow the onscreen instructions to unlock your Mac and reset your password. References: = Apple - Support - Manuals, Find My User Guide, iCloud Account Recovery; Apple Device Support Exam Prep Guide, page 6.