Into the Forge is a mystery as well as an achievement that requires the player to complete it. Access to Thalmund's Forge is gained during the mystery.
If you destroyed any of the keys to save space, they can be retrieved from Zanik only after starting the mystery. They do not need to be re-obtained.
You can observe the northern forge door and compare the deadbolts (large cylinders on the outsides of the doors) to the Labelled Forge Door Keys image below to check which keys you have already placed in the door. Any of the deadbolts remaining are still locked and thus the key has not been offered to the door.
- Find all 12 keys at the Warforge dig site:
- Dorgeshuun key, already in the northern Forge Door; no need to obtain it.
- Horogothgar key, initial crucible arena excavation site.
- Drogokishuun key, after excavating the barricade, which requires 83 Archaeology, and Entering "Tunnels Entrance" from the top level, choose both "left" and "right" in the dialogue box.
- Huzamogaarb key, excavate anything (including the material caches of vulcanised rubber and Yu'biusk clay) in the area of the goblin dorm debris in the South goblin tunnels. The vulcanised rubber requires level 76 Archaeology, while the Yu'biusk clay and goblin dorm debris require 83 Archaeology. Boosts can be used to excavate any of these.
- Saragorgak key, excavate anything (including the material caches of Mark of the Kyzaj, warforged bronze, and malachite green) in the area of the Big High War God shrine in the North goblin tunnels. The material caches requires level 76 Archaeology, while the Big High War God shrine requires level 89 Archaeology. Boosts can be used to excavate any of these.
- Garagorshuun key, fish cavefish near the malachite green or eel swarms from the bridge over the green river in the middle which requires at least level 28 Fishing. The key is guaranteed if you haven't received it after catching 50 fish. 
- Thorobshuun key, mining any of the copper, tin, or necrite rocks to the south. The key is guaranteed if you haven't received it after mining 50 ores.
- Ekeleshuun key, go through south-east tunnel which requires at least level 35 Dungeoneering.
- Idithuun key, go through south-west tunnel which requires at least level 35 Dungeoneering.
- Narogoshuun key, go through west tunnel which requires at least level 35 Dungeoneering.
- Yurkolgokh key, in the south-west area, north of the goblin dorm debris, attempt to open the southern "Forge door" (of Thalmund's Forge).
- Rekeshuun key, go through north-west tunnel which requires at least level 35 Dungeoneering.
- Use all keys to unlock Thalmund's Forge, on the northern door.
- Pull the lever in the centre of the room.
Forge doors, with keys labelled.
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- patch 4 May 2020 (Update):
- The forge doors inside Warforge! will now list the keys you are missing to open it, as long as you've found at least one already.
- ^ a b Mod Rowley. Imcando keys not dropping from eels. Reddit. 28 April 2020. (Archived from the original on 29 April 2020.) Mod Rowley: "The keys from fishing and mining are a 100% guaranteed drop if you don't already have one after 50 successful catches/ores."