The Room of Self-Actualization

The Room of Self-Actualization is a heavily guarded room in the palace of Canterlot. It's gigantic ornate doors have only been used by a small number of ponies, including Celestia and most recently Luna.

The Purpose of the room
The main purpose of this room is to help a pony see the truth with their own eyes. Celestia found herself lost and saddened, unable to look at herself in the mirror for sending her sister to the moon. Once she had used the room, she discovered the truth about herself and came to terms with all she had done, allowing her to be a better pony.

When Luna used it she fought Nightmare Moon, who was given a living form to face her. Luna was determined to free herself of Nightmare Moon, finding her to be nothing more than just a piece of her past she'd rather forget.

The irony was that Luna actually needed to embrace that part of herself if she wanted to move on with life, upon realizing this she and Nightmare Moon became one, no longer two seperate minds, but one single intelligence.

Future Usage
Not even the sisters know if the room will have any further uses for now. Though Celestia does have a bit of worry over Sonic, due to the fact she's felt the malicious energy of his Dark form.

Trivia

 * The Room of Self-Actualization was named after Maslow's pyramid: hierarchy of needs whHyperbolic_Time_Chamber.gif at the top of the pyramid stands the highest of individual needs: Self-Actualization.
 * The Room of Self-Actualization was based after the Hypobolic Time Chamber from Dragon Ball Z, whom the author is a huge fan of.