According to legend, before the Battle of Milvian Bridge where he cemented his claim to the Roman Empire, Constantine the Great saw a vision of a radiant cross in the sky. Constantine was inspired by the vision and converted to Christianity; as he became the uncontested Emperor, his new religion became the official religion of the Roman Empire.
The tale of the Lamp of Constantine adds another wrinkle to this legend: supposedly, Jesus granted Constantine a piece of the sun itself as a reward for his conversion. Constantine placed this holy relic in a lamp bearing the intertwined Greek letters "Chi" and "Rho" in memory of his vision and in honor of Christ. The Lamp is said to possess the power to destroy demons and other creatures of the night with its holy light. In the modern era, several disparate groups are aware of the tales of Constantine's Lamp. In particular, vampires who know of this legend are quite leery of the idea of anyone, least of all a mortal witch-hunter, holding a piece of the sun itself. Various sources imply that the loss of this Lamp may have in some way allowed the fall of the Roman Empire.
The Toreador elder Katherine of Montpelier, shortly after awakening from a long torpor, related the tale to her descendant Carmelita Neillson. Carmelita was herself vaguely familiar with the tale through the writings of Hainault, though she was less convinced than Katherine that the Lamp might actually exist.
Several vampire-connected rivals of an archaeological bent are known to seek the Lamp: namely, the Giovanni ghoul Claudia Dunsirn (pupil of Guillaume Giovanni, who has recently vanished), the Toreador ghoul Marcella Pryce, and the Tzimisce Troy Chervenic. Claudia believes that the power of the Lamp would alleviate the Curse of Caine in its bearer, and believes that it may have found its way to Africa. Elsewhere, a letter by a Kindred author (signed "Nobstulling") refers to Guillaume's presumed destruction and suggests getting more information about the Lamp from Katherine. The letter's author indicates that she has formed an alliance of sorts with Marcella (but is willing to sacrifice her if necessary), and reiterates the need to stay ahead of Troy in the quest for the Lamp. Presumably, this letter was somehow discovered by a hunter of the Redeemer creed.
In Days of Fire, the unknown author prophecies the rediscovery of a piece of the sun (i.e. the Lamp) by one of the "Bright Shiners" in the end times. On October 1, 2003, WTOJ intercepted a message that was posted to Hunter-net.org by Joshua Matthews ("God45"), a Wayward imbued, writing "The sun is gone but I have a light"; a lyric to the Nirvana song "Dumb". This may have been a reference the Lamp itself.
From a metagame perspective, the Lamp of Constantine is often used as an Easter egg, recurring in books written by Greg Stolze.