The Destiny 2 Bound Manacle is a hidden item found in The Gatehouse necessary for completing the Essence of Insanity quest. But unlike Shadowkeep's other hidden items, like the Withered Plumes or Ethereal Charms, the Bound Manacle is actually the reward for killing a relatively easy miniboss near the Hellmouth zone.
It's an easy enough task that's worth doing because Essence of Insanity rewards you with the Love and Death legendary grenade launcher—and lord knows you can never have too many ways to blow something up. So if you're hunting for the Bound Manacle in The Gatehouse, this guide will show you where to go.
Destiny 2 Bound Manacle and The Gatehouse Location
Fortunately, this one is pretty easy to find. From Sanctuary, head to the northern part of the Moon into an area called Hellmouth. Keep going until you're at the very northern part of the large pit and there you'll see a large structure with openings on either side that reveal a passage lower into Hellmouth. Follow this passage and you'll soon find yourself in The Gatehouse—but you're not quite where you need to be.
From where you enter The Gatehouse you'll see an orange glow marking a passageway that opens into a hall with white crystals hanging from ropes. Go through the hall and make a right and the room you enter into is where the miniboss spawns. Kill some of the Hive there and Erath'ur Eternal Blade should spawn—just look for the giant Knight with the big sword and energy shield.
Taking down Erath'ur isn't too hard, but his energy shield isn't attuned to any specific element so there's no quick way of taking it out. You're just going to have to chip away at him while keeping your distance and making sure to not get overwhelmed by other Hive in the area. Bring a friend or two to really stack the odds in your favor.
When Erath'ur falls, he'll drop the Bound Manacle, the last objective needed to complete the Essence of Insanity quest and get the Love and Death legendary grenade launcher.