In this Eltariel mission, you'll be defending the three local haedir from corruption (you need to have already cleansed them for this mission to become available). A ringwraith stops by to taunt you about it, and then you have just under a minute to race to the nearest tower.
Summon or steal a caragor, and do just that.
On arriving the timer will stop, and you'll need to wipe out the local orks, plus their low-level captain. Now's also a good time to start working on the optional objective of mounted kills. Glaive hits will do, but mounted execution are a sure thing, and they'll restore your mount's health.
The moment the last orc dies, you'll have one minute and twenty seconds to make it to the next haedir, so mount up and make haste. As before you'll want to wipe out the local orcs, but note also the spare caragor here, in case the first has succumbed to battle damage
Once this lot is downed, you have a minute thirty-five to reach the final tower.
This time you'll face the ringwraith himself. As always in nazgul encounters, you'll want to take him out first. He'll use the same attacks you remember from Minas Ithil. A good strategy is stay away from the main mob and wait him to come to you with one of his tele-stabs.
Counter and pour on the damage. Repeat until he's broken enough to Banish.
Despite the enemy counter, banishing the Ringwraith immediately completes the mission.