In Shadow of War: Blade of Galadriel, to upgrade the Light Bringer's Dagger, the challenge is to kill a captain with stealth, while invisible:
However, it is completely unclear how to be "invisible". I have tried killing captains while hidden in bushes, aerial stealth kills, and sneaking up behind captains to kill them, but none of it seems to count as being "invisible". I also can't find anything relevant in any of the character skills, and I have unlocked them all.
How do I become "invisible" for the purpose of completing this challenge?