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Can't Adrasteia have powers over the Misthalin kingdom too? Don't forget Saradomin has powers over Lumbridge, if not Misthalin, as Duke Haracio came to Saradomin, not Roald, for help when Lumbridge was broke
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She is the empress but for some reason is called a 'queen'. The kings are second only to the empress. The duke is in charge of Lumbridge because it is his lands. People built their homes and businesses around his castle, so he is in a way their boss and renter. The duke is a rich landowner in Misthalin that still serves the Misthalin crown. All decisions of the Misthalin Kingdom still fall under King Roald, but Adrastea has the power to overrule him. Adrastea is also in charge of Asgarnia and Kandarin. Think of it like America, there are several governors for each state but there is only one president.