Its release wasn't without a hitch, but Destiny 2: Black Armory's Bergusia Forge is finally here. With all four Lost Forges now available, the Mysterious Box quest can be completed in order to earn the Exotic sniper rifle Izanagi's Burden, AKA the railgun. It's a fairly lengthy and grindy quest but you'll earn a lot of powerful gear along the way, and the railgun is well worth it, so let's go over the Mysterious Box step by step.
Where to get the Mysterious Box
Before you can open the Mysterious Box, you'll need to find the darn thing. Head to the Volundr Forge in the EDZ, but instead of going to the central Forge itself - that is, the metal doohickey where you start the activity - head to a cave located below the area's entrance. Basically, when you reach the broken bridge, drop down, turn around, and look to your right. You'll find the Mysterious Box inside a Black Armory crate in the back of this cave. Refer to this GIF if you get lost:
How to get the Fishhook, Butterfly, and Hand Keys
The first step in this quest is to obtain four keys and open the box. Each key corresponds to a Forge, and to get them, you'll need to clear that Forge with the Maximum Temper buff active. To trigger this buff, you have to destroy two Black Armory drones which spawn at the start of wave two of the Forge. The Black Armory key requires some additional legwork, so for now here's how to get the Fishhook, Butterfly, and Hand Keys.
Mysterious Box Fishhook Key: Fire up the Volundr Forge in the EDZ, complete wave one, then head to the left, coastal side of the arena. Bring a sniper rifle. At the start of wave two, the first drone will spawn floating above the grassy mountain off the coast, and the other will be floating in the cave to the far right of the arena around some stalactites. You can see both of them from the cost, so shoot them down - and do it quickly, as they will despawn - to trigger Maximum Temper, then finish the Forge as normal. A bonus Black Armory crate containing the Fishhook Key will spawn near the Forge core. Refer to this GIF if you can't find the drones:
Mysterious Box Butterfly Key: Head to the Izanami Forge on Nessus. Bring a sniper rifle, finish wave one, and shoot the drones to trigger Maximum Temper. The Black Armory drones in this Forge spawn above the Vex spires on the two smaller islands. Luckily, both are visible from the central island, so post up there at the start of wave two. Again, a Black Armory crate containing the Butterfly Key will spawn near the Forge core once you finish the activity. Refer to this GIF if you can't find the drones:
Mysterious Box Hand Key: Go to the Gofannon Forge this time, and get ready for more of the same. The Black Armory drones in this Forge are close together and easy to spot, plus you don't need a sniper rifle this time. Stick to the back of the arena near the ship wreckage when wave two starts. The first drone will be hovering near the top of the wreckage, and the other will be to your left toward a spawn cave. Shoot them, finish the round, and loot the crate for the Hand Key. Refer to this GIF if you can't find the drones:
How to get the Black Armory Key and open the Mysterious Box
The Black Armory Key is also tied to a Forge, but you've got to do some grinding first. Start by visiting Ada-1 in the Tower. She'll give you the Black Armory Key Mold and tell you to kill Watchers in the underbelly of the Leviathan raid. This can be done completely solo, and it's a lot simpler than it sounds.
How to farm Watcher Lenses: Load into the Leviathan and drop down to the platform under the bridge you spawn on. Head to the control room at the end of this platform and you'll find six levers. These levers will open the way to the Leviathan's underbelly when pulled in the following order: 1 5 3 2 4 6, with the levers numbered 1 to 6 from left to right. Put on high mobility armor if you can't get the levers to activate - you're probably just moving too slowly - and slide into the levers to give yourself a speed boost. Refer to this GIF if you're having trouble:
Once the path is opened, head outside and go up a few levels until you see two large pipes heading into the bowels of the ship. Follow these pipes to a room in the back, pull the lever inside to activate the nearby air vent, and ride the vent upward. Head down the small nearby hallway and follow the linear path until you reach another door with a lever, which leads to the Engine Room. Pull the lever to open the door. Inside you'll find four Watchers, little sentinel robots which act as the Leviathan's alarm system. Once you kill one Watcher, the others will sound the alarm and you'll need to dispatch a wave of Cabal before you can kill the rest. So, you know, kill everything. Here's a quick look at the path to the Engine Room:
The Black Armory Key Mold says you need to destroy every Watcher in the Leviathan, but you don't; you just need to kill the four unfortunate Watchers in the Engine Room six times. So, once you clear the room, head outside and pull the lever again to respawn everything. Repeat this process until you've collected all 24 Watcher Lenses.
How to farm Glimmering Amethyst: Next, you'll need to collect 200 Glimmering Amethyst. This can be obtained by completing Public Events, opening planetary loot chests, and finishing Strikes. Grinding Heroic Public Events is the fastest way to get Glimmering Amethyst because they don't take long, they award 10 Amethyst each, and you can loot planetary chests between events. So, head to the Public Event-rich EDZ and get to farming. Or, if you'd prefer, you can grind 10 Strikes, if you don't mind investing a little more time.
Mysterious Box Black Armory Key: This key is in the 650-power Bergusia Forge, but we're still following the same plan as before. Get to wave two, shoot the drones, and finish the Forge. The first drone will spawn on the left side of the mossy ruins in the back of the arena, and you'll find the second drone hovering around the large red pipe on the right side. Refer to this GIF if you can't find the drones:
Loot the crate at the end of the Forge, pop all your keys into the Mysterious Box, and you'll receive a Mysterious Decryption Device and a Mysterious Datapad. The latter has its own independent questline, which you can follow with our Destiny 2: Black Armory Mysterious Datapad quest guide, so we're going to focus on the Decryption Device. Bring this to Ada-1 and she'll tell you about an Unidentified Frame, which is the next step in the quest.
How to forge the Unidentified Frame
This is honestly the simplest step in this process, but it's also the hardest. To start work on the Unidentified Frame, you'll need to get an Obsidian Crystal by turning in a rare Black Armory bounty. You randomly receive these rare bounties when completing the daily Black Armory bounties sold by Ada-1, so just do every bounty you can and pray. Note that you have to receive the rare Black Armory bounty on the character you used to do the Mysterious Box quest.
Once you finish a rare bounty and get the Obsidian Crystal, you'll need to get an Ascendant Glass Shard by finishing the Shattered Throne dungeon in the Dreaming City. The Shattered Throne is only accessible every three weeks, when the Dreaming City's Taken curse is at its worst. It is live at the time of writing, but you've only got until Tuesday, January 15, so act fast. If you're reading this in the far, far future of January 16, sorry, but you'll have to come back on February 5 (or the next applicable Tuesday if you're reading even farther in the future).
Next, you'll need to complete a special, 650-power version of the Pyramidion Strike. The good news is that this Strike comes with the Heavyweight and Solar Singe modifiers, meaning everything inside can be absolutely eviscerated by Whisper of the Worm and Sleeper Simulant. The bad news is it's 650 power and it also has Extinguish, meaning if you wipe, you'll have to restart the whole thing. Oh, and your gear is locked. So, you know, get in there and don't die. You should be able to spawn kill the boss if your fireteam is running a mix of Whisper and Sleeper, but be careful that you don't let him live after destroying his head, otherwise you'll have to deal with a bugged, indestructible death bot for several minutes.
You're now the proud owner of an Unidentified Radiant Frame. Fantastic. Time to do another mission. It's called Lock and Key and it's in the EDZ. Polish it off and you'll receive a strange item called Not a Weapon (it is not a weapon).
Head back to the Tower, talk to Ada-1, and you'll receive the Exotic sniper rifle Izanagi's Burden. Congratulations! Go railgun some things to hell and back.