The Devil's Heart, or Heart is an incredibly ancient artifact that has shaped the histories of a number of Alpha and Beta Quadrant cultures during it's long existence. Appearing as a small stone, the object was an extremely advanced device.
The Heart was originally created by the same race that built the Guardian of Forever. They intended to send the stone on a journey to a different galaxy, where the stone would grow into a second Guardian of Forever. However this race died out before they could see that the stone reached its destination, and the stone was unable to complete its journey.
Over time, the Heart ended up in the hands of a number of different races, and was known by a variety of names. The Heart crash landed on the surface of the Tkon home world, and what were the primitive Tkon people used the Heart's power to create a vast interstellar empire before their empire collapsed in on itself. The Heart also ended up with the Iconian people until their home world was destroyed 100,000 years ago. It also figured in early Vulcan history - Surak's family had possession of the Heart until it was taken by a rival clan. When the leader of the rival clan left Vulcan, he took the Heart with him to Romulus - thus the Heart became part of Romulan history. The Andorians accquired the stone when the ship carrying the Romulan Emperor crashed on their world - an Andorian healer used the Heart's power to stop a plague which was devastating the Andorian people. It was later given to the Ferengi, who traded it with primitve Klingons. An ailing Klingon Emperor gave the stone to his slave, who used the stone to found an artist's colony - and she entombed herself alive with the Heart.
In 2368, the Vulcan archaeologist T'Sara rediscovered the Heart. This resulted in a deadly race among the races that had held the stone before to posses the Heart and the powers that the Heart possessed again.
Captain Jean-Luc Picard discovered the Heart on a wrecked Ferengi ship. He was able to see the Heart safely though a wormhole to a distant galaxy, where the Heart would someday find a suitable planet and grow into a new Guardian of Forever.