How to unlock Mac password?
Boot to internet recovery and wipe / erase the drive. At that point all FileVault data and system are gone. You then can run the installer and drop a new OS on the Mac. 

As long as there is not a firmware password (which you shouldn’t have since that prevents internet recovery boot and other changes to the OS boot) you can run the Mac in target disk mode and erase the drive or boot to internet recovery or recovery HD (external) and erase.

Here are the official steps for an erase install of macOS:


https://support.apple.com/kb/PH25649