An artifact most Giovanni would give somebody's soul for, the True Vessel appears as physically a small earthenware jar, holding maybe a pint of vitae. The jar is sealed with beeswax and a curse and can't be opened by any force known to anyone currently familiar with it.
The matter was brought to the Giovanni's attention by the Capuchin. It is believed to hold a small amount of Cappadocius' blood and soul sealed by Japheth and Constancia, which Augustus Giovanni would really like to get a hold of. The True Vessel was rumored to have been hidden in the Erciyes library but lost when Claudius Giovanni ordered the burning of the temple-fortress.