The Elements of Harmony are six supernatural artifacts representing subjective personality aspects conducive to maintaining strong interpersonal bonds. They are arguably the most powerful force in Equestria, though the extent of their power is largely unspecified.
Thousands of years ago, the Elements of Harmony were found and taken from the Tree of Harmony by Princess Celestia and Princess Luna. However, doing so had the potential to release the Everfree Forest and destroy Equestria. Each artifact represents an element of friendship and can only work if wielded by one who possesses the corresponding trait and if all six are used in conjunction. They have been wielded on rare occasions, particularly during events marked by danger and anarchy as a last resort to restore and enforce the balance of peace and order. However, the effects of their use are generally temporary and non-lethal, usually a form of banishment or imprisonment.
During the Saddle Arabian War, Twilight Sparkle tried to relocate the Tree of Harmony, still containing the Elements, to a safe location outside the Everfree. She found she could not, so instead erected a military base around its cave in order to safeguard it. This military buildup attracted the Saddle Arabian's attention, prompting them to attack the Everfree at the Battle of Red Oat. The Saddle Arabian coalition, led by several mercenary dragons, succeeded in destroying the Tree. The Elements survived.
The six Second Era Element bearers realized, to their wonder and dismay, that they had stopped aging. Preliminary arcane research speculated that ponies tied to an Element cannot die of old age until it is relinquished to another, though bearers can still die from physical violence.