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You need to place four heroes, but you can't use Arantir! To free a hero from prison or visit the elders of Silanna, or to capture a lot of mines, you need to complete the ghost's quest (near the garrison to Holyflame, he asks you to kill his murderers :D). Then you will be able to send ONE hero through ONE portal; be sure to save! If you decide to visit the prison, exit through the portal with Valeria; Arantir can teleport to the city.