Destiny 2's Necromantic Strand is an item you'll need to find in Shadowkeep's main quest.
As you explore the Moon, you'll pick up a variety of 'Essence' quests, with a variety of Moon and Nightmare-related tasks.
One of these is Essence of Brutality, which you'll pick up after completing the 'Beyond' mission, with one of the steps is finding the Necromantic Strand within the Hall of Wisdom.
It's a location not on the map, so you might need some pointers of where to find it.
Where to find the Hall of Wisdom in Destiny 2
First, let's get to the Hall of Wisdom, which is a location deep underground within the Moon.
To get there, start at Archer's Line, and leave via the north-east exit (not to be confused with the passage that takes you to Hellmouth). To find it, you need to find a domed building:
Head through the building and on the other side, you'll see the entrance to an underground location ahead.
Head through the green-tinted doorway, drop down in the area below, and take the corridor round to arrive at the Hall of Wisdom.
Where to find the Necromantic Strand in Destiny 2
As soon as you enter the Hall of Wisdom, look right to see a red-tinted area with a glowing spiked object hanging above.
Head there, and just at the other side of this object you'll find what you're looking for, against a wall.
Activate the on-screen prompt that says 'Pick Up Weapon Part' to receive the Necromantic Strand.
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