Nier Replicant is stacked with quests, but only a few of them are vital to collecting all of the game's weapons to unlock any endings beyond Ending B. If you're working on finding every weapon, then you'll need to finish A Shade Entombed. It's not actually the quest that rewards you with a weapon, but it is a prerequisite for finishing Disturbing the Sleep of Kings, which will reward you with Labyrinth's Song.
A Shade Entombed isn't something you can immediately do once in Facade as younger brother Nier, during the first part of the game. You'll have to wait until you return to the area in the second half as an adult, then you can complete the questline for a reward of 30,000 gold. While it's no weapon, it will give you a hefty chunk of change for buying what you need from shops. Before beginning the quest, you'll also need to have finished Facade's questline in the second half of the game and have obtained the Cerberus Key Fragment. You'll receive this after the boss battle with the wolf Shade.
How To Complete A Shade Entombed Quest
First things first, you'll need to head to Facade in order to pick up the quest, A Shade Entombed. After you arrive at the front of the desert city, head to the gates and speak to the guard standing on the right side of Facade's entrance. Accept the quest, and you're ready to begin working towards unlocking Disturbing the Sleep of Kings.
The guard clues you in on what's going on, there's trouble in the Barren Temple. You'll need to head there and take care of the giant Shade running amock in the temple after bandits released the creature inside. So, leave Facade and make your way back to the Barren Temple - the area where you saved Facade's King in part one.
Now, you can either head to the back of the temple to challenge the big Shade and wrap up the quest, or you can comb through room by room and kill the little guys. We recommend going ahead and doing the latter, because it'll save you time when completing Disturbing the Sleeping Kings. So, head through each room that once contained a puzzle and take out the smaller mobs there, getting in a little extra cleanup before you finish this quest up.
Once you're done preparing for your next adventure here, you can run to the back of the Barren Temple and kill the big Shade waiting for you. The fight isn't particularly tough, and once you're done Nier will suggest taking the surviving bandit back with you to Facade. Head to the back of the room, investigate one of the men laying on the floor, and you'll exit the Barren Temple.
After one more brief encounter, the bandit will give up and you'll haul him back to the city. The guard will give you 30,000 gold and you can now begin Disturbing the Sleep of Kings and work towards obtaining the Labyrinth's Song.