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Sapience said today on the official forums that Update 5, which will follow Isengard and release later in the year (late Autumn, early Winter), will contain 5 new instances.
We are currently preparing five new instances for release post Isengard.
Rise of Isengard will be released on September 27 along with a 24 Player raid. There will be 3 Three player, 1 Six player and 1 Twelve player instance released in Update 5, the first update after the release of Rise of Isengard. These instances will be free to any player who has pre-purchased the Rise of Isengard expansion or who purchases after launch via the Turbine Web Store.
He replied after clearifying the last sentence, saying that means anyone who preorders it or buys it afterward via the same way preorders are done will get it free.