This story is a spinoff of Harry Potter and the Prince of Slytherin, but can be understood without reading that.
Dear Candidate,
Congratulations! It is with great pleasure that I offer you a position at the one and only Department of Mysteries. You have been given this amazing opportunity in recognition of your astounding magical and intellectual talents, but know that if you do mess up, there will be significant consequences for yourself and for Magical Britain. Please review the attached 1391-page training manual before our orientation meeting next Tuesday at You Know Where.
Now, the majority of this year's recruits have picked the Mind, Time, Space, or Death Departments as their first choice. While we understand the surge in interest following certain, ah, anomalous incidents recently, your assigned Department will depend upon your strengths, DoM personnel shortages, and more. Still, don't fret if you aren't sorted into your desired department initially; researcher duties are rotated on a regular basis to ensure familiarity with all aspects of the Department's research. Also, while we aren't all as glamorous as the Big Four, all ten DoM departments engage in fascinating, terrifying, and important research. I myself work primarily with Muggles, and I have never regretted my choice to specialize in experimenting upon our nonmagical and lesser counterparts.
If you were one of the twelve candidates who indicated you wanted nothing to do with the Department, we are sorry for the mixup that resulted in you receiving an orientation letter, and we assure you that you will remember nothing of this incident come tomorrow morning. The Department may make mistakes occasionally, but we are nothing if not brutally efficient at fixing them. I think I can still hear Number 9's screams.
To the rest of you: Welcome to the Department of Mysteries. There will be research and knowledge and magic and only a little bit of screaming.