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).