Roman Aquila is a quest item used during the quest Ghosts of the Empire. If you side with the Remus faction, Iovita will ask to defeat the Romulus faction and to take this standard from them and and hand it to her, so she can lead the Legion with the remaining followers.
In case the item is acquired and not used during the mentioned quest, it can be taken to Astridr in Skjern to gain five prosperity for your homestead.
In-game description[]
An ancient eagle standard of a Roman legion, cast in gold.