The Artifact of Lorta was a magic amulet that acted as a prison of the demon lord Karkull.
With a certain incantation, a magic-user could draw Karkull into the tablet. However, Karkull also appeared able to manipulate the appearance of the tablet to some degree, and trick some gullible person into reading the spell to release him. Probably for this reason, the tablet was hidden away by some ancient civilization inside an innocuous clay jar. At some time, the jar was discovered by archaeologists and kept in a storage room at Metropolis University.
In the 20th century, the jar was broken, and the tablet discovered and read, by a sneak thief, who inadvertently released Karkull. Karkull crushed the tablet to powder, but it was magically reconstructed by Dr. Fate, who used it to imprison the demon again. What finally became of it is unknown, but Fate may have stored it for safekeeping in his tower.