How does one become invisible in Shadow of War: Blade of Galadriel?

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?

In addition to some gear that provides situational invisibility, Her version of Elven Wrath also gives invisibility. One of her updgrades to Elven Wrath also extends that invisibility.

I used that invisibility to good effect on an otherwise particularly annoying Olog Captain.