How to re-create the ##MS_PolicyEventProcessingLogin## principal I'm getting a bunch of errors in my MS SQL Server logs about a missing ##MS_PolicyEventProcessingLogin## principal. I went to check the master database and yes, it is indeed missing. I don't know how this happened.

How do I recreate this principal?
Any account enclosed in "##" are certificate based accounts and you will not be able to simply create them as you would other logins/users. If you have a backup of the master database prior to the accounts disappearing you could might try restoring the database to get them back. Other than that I suspect you will need to reinstall or try rebuilding the master database.

Read through these two articles (here, here) for rebuilding the system databases. I would suggest taking a backup of the current state of the system databases. Script out all the logins you have in master as you will loose these.