Sadly, the Destiny 2 Necromantic Strand is just a collector's item at this point since the weapon it unlocks, the Loud Lullaby hand cannon, has been sunset. There's nothing wrong with filling out your collection for completion's sake, however, so here's a quick guide on where to find the Necromantic Strand in the Hellmouth section of the Moon.
How to find the Hall of Wisdom
If you've already obtained the Destiny 2 Horned Wreath, then you've probably passed by this item's location already. The Hall of Wisdom is found along the same path as the Chamber of Night, and it isn't too deep in the Hellmouth. From the main landing zone, head west until you reach a hill that leads to an old lab. Head through it and enter the Hive temple in the back.
Keep descending until you reach a well-lit area with almost an almost sepia hue to it. You should see a big-old chandelier-type fixture in the middle of the room with some crags and spires below it. The Destiny 2 Necromantic Strand is along the back spire underneath the chandelier. It's hard to see on the ground, but you should see a marker leading to it, and the range for the pick-up button prompt is pretty generous.
Now that you've got the Necromantic Strand, you can polish off your Essence of Brutality and forge the Loud Lullaby hand cannon. And now that you've gotten it once, you won't need to collect it ever again, so future Essences will be easier to complete. Thank the Hive gods for that.
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