Along with Destiny 2's move to Steam is a new expansion, Shadowkeep, and part of the new storyline, players will have to cleanse various essences for Eris Morn. One of these essences is the Essence of Insanity, and that requires you to find the location of the Bound Manacle.
Unlike the other Essence quests, though, there's no way to find the Bound Manacle out in the environment, instead, it's obtained as a drop from a particular enemy.
To find it, head to Hellmouth, located on the Moon, and you'll come across a green pathway. Go inside, then take a left turn, and run through the path right into the dungeon.
After about a minute, you'll reach a location known as The Gatehouse. There's a lot of enemies here, including a mini-boss known as Ethrathur External Blade. Kill the boss, while keeping an eye on the other enemies spawning around you.
Once you've defeated the boss, he will drop the Bound Manacle item, which will allow you to continue with the Eris Morn quests.