Celestia's Necklace

The Celestia's Necklace is a powerful artifact that was designed as a safeguard in case of an attack on Stable 13.

Properties
When the Stable is attacked, the Necklace creates an impenetrable barrier between the danger and the Stable.

The barrier also covers the Stable's Alternate Entrance which is located several miles from the Stable on a mountain.

The barrier prevents anything from getting in or getting out.

The necklace seems to activate automatically as soon as it senses any grave danger to the Stable.

The barrier will not dissipate until the danger has passed, hinting that it has a degree of omniscience.

The necklace has to be worn by a Alicorn for the barrier to take effect, and the gem in the center glows bright purple when the shield activates.