Yelough Anix Tunnel is the lone tunnel found in the Consecrated Snowfield — the only one in the whole mountain range, in fact, which isn’t very realistic. This is an important location to visit, since it’s full of end-game level upgrade materials. It’s also unique as far as tunnels go, since it’s the only one covered in ice — unfortunately, this feature makes finding the smithing stones on the walls pretty tough.
After riding the elevator down and activating the site of grace, head north into the tunnel. You’ll enter a small chamber with a couple dead miners — continue northeast into the next segment of tunnel. In the following chamber, walk slowly so as to not fall into the pit by accident. To the right (north) is a shortcut door you’ll open later. For now, walk around the right edge of the pit to the far wall, then look over the edge and drop onto the ledge with the dead miner. From there, you can drop once more to the ground.
As you land, an icy miner will defrost nearby, as will another on the northern side of the room. Backstab the first one as it awakes, then go handle the second miner. On the wall near the second foe is the first of many hard-to-spot Smithing Stone .
Grab the smithing stone, as well as the x3 Freezing Grease from the corpse in the center of the room, then head south into the tunnel. It will wind a bit and take you deeper into the mine, and then you’ll come out on a wooden platform. On the right is a miner — attack it before it can defrost, then continue east on the wooden walkway and backstab the miner in the corner. To the north is another miner who is probably throwing rocks at you — from the corpse, jump across the gap and kill the rock-flinger, then jump back, pick up the Golden Rune , and go down the ladder.
Once you get off the ladder, turn to the south, where you’ll see another Smithing Stone  on the floor against the wall (see here). Grab it and then look to the northeast, where you’ll see a corpse with a Rune Arc — there are two frozen miners to the left of it, so backstab one while they defrost, then kill the other and loot the corpse. Head back to the ladder, and from there go northwest.
Around the next corner on the left, an Onyx Lord will spawn — you can choose to fight him, or you can ignore him and just pick up the rest of the items in this room (he won’t be able to follow you very far). To the west, in the alcove the Onyx Lord spawned in, are two Smithing Stone . More importantly, there is an Ancient Dragon Smithing Stone in the northern corner of the chamber, so don’t miss it. You can then run to the ladder and climb it, then jump the gap as pictured above and continue north into the nearby tunnel.
In the next room, hug the right-hand wall and sneak up on the miner sitting by the fire. Backstab him, then pick up the x6 Explosive Stone Clump from the corpse, and snag the Smithing Stone  from the western wall.
Next, climb the ladder in the center of the chamber. Once up on the platform, turn around (face south) and kill the miner who’s preparing to chuck rocks at you. Then face north again and kill the miner on the far wall, grab the Smithing Stone  from the nearby ice, and then climb the ladder.
On the next platform, go north, jumping up the rocks to enter the tunnel. This tunnel will bring you to a large chamber, where another Onyx Lord will spawn. While he’s still spawning in, sprint past him and into the next room, then continue west up onto the snowy ice — on the left here is the door that serves as this dungeon’s shortcut. Open the shortcut, and now you can face the Onyx Lord without fear of having to redo the whole dungeon.
Loot claimed, you can head back to the room with the shortcut door and open the large wooden gate to face the dungeon’s boss. This boss is a copy/paste of Astel, Naturalborn of the Void — the only difference is that the boss in this dungeon creates fake versions of itself when it teleports — only the real, more solid-looking boss can grab you. The rewards for the boss in this dungeon are the Meteorite of Astel sorcery and 120,000.