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A quick summary better than what I can do.

http://warhammeronline.wikia.com/wiki/Border_Princes
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I think the term covers anyone who rules a large amount of area in that area so anyone from the ruler of a small group of people to a large outlaw group. Also I don't think the border princes are restricted to normal humans, they also contain Ogres, Chaos warbands, Vampires and any other creatures that don't really fit in anywhere else.

Don't take this all as fact- I think it's correct from what i've read but im not 100% sure.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/259205/1018655.page http://www.dakkadakka.com/dakkaforum/posts/preList/259205/1018655.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/259205/1018999.page http://www.dakkadakka.com/dakkaforum/posts/preList/259205/1018999.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/259205/1023363.page http://www.dakkadakka.com/dakkaforum/posts/preList/259205/1023363.page GMT