Tree of Harmony

The Tree of Harmony is a tree located in the Everfree Forest near Princess Celestia and Princess Luna's old castle. The tree carries the Elements of Harmony and in the center lies Twilight Sparkle's cutie mark. Near the roots of the tree are Celestia and Luna's cutie marks.

Princess Twilight Sparkle - Part 2
In the second part of season four's two-part premiere, it is revealed that Princess Celestia and Princess Luna discovered the Elements of Harmony embedded in a tree, called the Tree of Harmony, in the Everfree Forest. Each element is embedded in one of the tree's branches, with the element of magic in the tree's center.

The tree and the elements were discovered during the reign of Discord thousands of years before the events of the episode, and were the only means the princesses had of defeating him. According to Celestia, the tree controls and contains all that grows in the Everfree Forest and will continue to do so even without the Elements in it.

During the two-parter, it is discovered that the Tree of Harmony has lost all of its magic, allowing Discord's "plunder seeds" to grow all over the Everfree Forest, slowly killing the tree. By the end of Part 2, Twilight and her friends give up the Elements of Harmony and return them to the tree, recharging its magic and returning the forest to normal.

When the Tree of Harmony regains its magic, a flower sprouts at its roots. The flower blooms to reveal a box with six locks, requiring six keys to open.