What it is

The Iron Crown of Lombardy is a pedestal item of the Holy Roman Empire. The in-game line says the iron circlet inside it was beaten from a nail of the cross. The historical-artifact row marks Royal Court.
It sits with the Reichskrone on the empire. Henry IV is the 1066 emperor. Frederick Barbarossa is the 1178 emperor.
How to obtain it
Take the Holy Roman Empire, or start as its emperor on the 1066 or 1178 bookmark. The crown is a court pedestal, so it is placed in the throne room. A pedestal uses one of the four pedestal slots.
On a bookmark with no Holy Roman Empire title, a missing historical artifact can turn up through an Adventure inspiration. Royal Court is the expansion the artifact row names. Sponsoring that inspiration also accepts Roads to Power.
What it changes
The Iron Crown supports monthly renown, monthly prestige, vassal limit, and monthly development growth in the realm capital. It also supports Italian opinion, Lombard opinion, and court grandeur.
The amounts stay on the tooltip. Court grandeur applies when the piece is displayed. Without Royal Court the throne room that displays it is closed.
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