Released at GenCon 2011; The Massing at Osgiliath is a print-on-demand encounter deck (so; not a full expansion like the monthly decks) that is designed to be more difficult the more players you have. A POD deck is now available through the Fantasy Flight website for $15.

Description from Fantasy Flight:

With the forces of Mordor hard on their tails; the heroes will need to work together to survive Sauron’s vanguard and find their way across the Anduin. But it won’t be easy. The four-part quest for The Massing at Osgiliath pits your heroes against a brand-new encounter set; filled with deadly enemies; including the Black Captain of Sauron’s forces; the Witch King.

The Massing at Osgiliath contains 41 encounter cards and 4 quest cards (i.e.; only bad guy cards) While no official difficulty rating has yet been assigned; Fantasy Flight representatives at GenCon said it was an 8 or 9 and it is truly difficult (especially for four players using the prebuilt starter decks).