(Spoiler) Witch king + Eltariel side mission disappeared?

In Act 2, after regaining the new ring of power I briefly saw a mission on the ground which mentioned a ring wraith (probably had something to do with Eltariel, the elf lady who saved us from domination in Act 1).

Since I was very interested in how fort sieges go, I decided to complete that quest first. After the fort was conquered, the nazgul mission was gone.

I was wondering if the quest will reappear later or am I screwed for the playthrough (one of the skill customizations mentions the completion of Eltariel missions)?

Update: The first paragraph here http://www.eurogamer.net/articles/2017-10-10-shadow-of-war-guide-walkthrough-tips-4823?page=12 mentions

You may end up unlocking some of them in a slightly different order to how we've listed them as they're dependent on some free-roam activities and quests in other sets, and we'd actually suggest leaving this set of quests until after you've completed Brûz, Carnán, and Gondor - the extra skills and resources unlocked through those will come in handy, and it eliminates any danger of rendering them incompletable.

Not really sure what this means, is this possible to make these quests incompleteable then?

After taking the first fort, the mission disappears from Nurn indeed. But a new one appears in Minas Morgul, and after that is completed, the one in Nurn is playable again.

Tip: The easiest way to check the available quests is from the actual "Quests" menu found below the "Army" and "Inventory" menus.