How do you get the Eye of Aman'Thul in WoW Legion?

Gul'dan drops it, so it's the Nighthold raid, right at the end. You need to be at the point in the Suramar storyline where Khadgar sends you after it, then any kill of Gul'dan hands over the quest item, and I'm fairly sure LFR counts. At max level you can walk the whole raid solo in an evening, the hard part is just finding the entrance in Suramar City.