Book 14, and eventually leave the mines to explore Lothlorien on the other side, including Galadriel's garden and well. This is where you will meet up with The Fellowship once again.
There will be ten new areas of Eregion, nine of which will be accessible to anyone. The final area will be the trip to the actual Hollin Gate itself, which will only be available to those with the expansion. At this point you will fight the Watcher in the Water, but you won't get very far with him. While mending your wounds, the Dwarves will let you know that you'll need some more powerful weapons to defeat it, which brings us to the Legendary Items.
Follow along with an explanation of these Legendary Items>>