This is the mission to rescue Baranor, after his capture in Wages of War. You start with objective to find him, and the bonus goal of freeing 9 Gondorians.
There are more than 9 to free, which is handy, because they're not clearly marked on your map. Go ahead and free any you happen to pass on your way to Baranor.
As soon as you get close to the man himself, you'll be ambushed by two captains. But also, Idril shows up with reinforcements. Your goal is to take the two captains down.
Note that Idril can't actually die here, but she can be hurt to the point of needing revival, so if you value her sword, keep an eye out.
When the two captains are disposed of, free the two no-named hostages here in the pit, then double back to the ruins to get yourself to the 9 required rescues. There's a clump of three in the middle of the ruins, dead north of the local ithildin (or where the ithildin used to be, if you've already collected it).
You can free more than the required nine if you like, though there's no acknowledgment of this in game. When ready, go back and free Baranor himself for a bit of resolution
This completes the mission, and the Gondor questline
It also unlocks the Iron Guard skill, which makes your summoned warriors tougher.