This post concerns an important event item and it may or may not contain spoilers [in this case the spoiler would be concerning gameplay and not much on story]
Proceed at your own risk.
Dragon Key #2 - The Mithril Key
aka: The Ruby Key and/or The Heart Key
Description: This key is made of the finest silver, a metal that is in fact more rare than gold. If it were to be valued, it could possibly purchase all of Ponyville and some of its neighboring lands. The handle is shaped like a heart with a perfectly smooth spherical red ruby in its center. The head blade also has a heart shape cut out of its center.
Quest: To acquire this key, you will need to find all the heart containers hidden throughout the world. Once you've done this, a secret bridge will reveal itself in a chamber which would lead you to another chamber where the key is held.