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Q: How do I play as Arnor? I can't find this faction in the campaign selection screen.
A: Arnor is an emergent faction that Eriador transforms into when certain conditions are met. These conditions are communicated to the player via the description of Aragorn's ancillary called Elendilmir which represents the crown of Arnorian kings. Simply mouse over the ancillary and read its tooltip.
These objectives are as follows: capture or hold: Hobbiton, Michel Delving, Longbottom, Bree, Staddle,
Barrow-Downs, Amon Sûl, Annúminas, Fornost Erain, Athilin (new in 3.0!!!), Hoarwell, Tharbad,
Coldfells. Furthermore you need to establish an alliance with Gondor, build the House of Kings in Annúminas (only possible If you have upgraded this settlement into a large city) and finally Aragorn must be alive. If these conditions are met, then you will receive an event, which tells you can reform Arnor. Choose accept in case you want to play as Arnor.