There's quite a number of character and story ideas that can be pulled from Venice's history up to 1220 AD. Among them:
Doge Vitale (II) Michiel was killed in 1172 by a member of a mob for his gullibility in confronting the Byzantine Empire, losing a large portion of Venice's fighting men as well as ships to plague, and for likely bringing that plague back to Venice. Though past Doge's had been killed by Venetians before, it had not happened for over 200 years. The authories tried and executed the killer himself. Orders then came down to raze the killer's house and prohibit any stone buildings to be erected in the lot. This tradition was held to until the 20th century.
The events and significance of the Doge's death allow for either a magical or faerie ghost of the Doge himself or his killer. Whether or not any supernatural significance is attached to, or results from, the killing, given the length of time in which the order to build no stone buildings on the killer's plot was observed and the relevant recency of the event to 1220, the event offers good color that is easily within the public mind at the time.
Saint snatching Venice's history up to 1220 includes two to three claims of having taken saints from other areas and entombed them in Venice. At least one of these was a fake. Divine, Infernal, and even Faerie (because the stories of how the saints were obtained are, true or not, are a part of Venitian pride) stories can be based off of the interred saints.