Another day, another unholy ritual to muck up for the Dark Lord. Eltariel will lead you to the spot.
Your bonus objective is to free at least one follower before things get loud, but this is easily done. The rescue mechanism is just like freeing Gondorians, and all you have to do is find a prisoner that the Nazgul aren't actually staring at. Even if they are staring, you can still pull it off if you're quick.
When the fighting does start, you'll be up against two captains and two nazgul. These are standard nazgul, no special powers, but you still want to prioritize them as targets. If you like, you can free your other captains to hold off (and/or slaughter) the enemy caps, while you focus on the ringwraiths.
When all opposition is banished or dead, you'll need to free any friendly hostages you haven't gotten around to saving yet.
After that there's a bit of banter, and mission completes.
Completing this mission gives you the extremely useful Shadow Dominate spell- ideal for dominating a Broken warchief who's still surrounded by tons of pesky grunts