Location
The Yelough Anix Tunnel is located in the secret area of the Mountaintops of the Giants, near Miquella's Haligtree. You'll need the two pieces of the Haligtree Secret Medallion to gain access to this area via the Grand Lift of Rold. Once there you'll wind up in a small dungeon called the Hidden Path to the Haligtree, which in turn leads to a vast ice field.
Follow the western edges of the snowfield to the northwest from the exit of the Hidden Path to the Haligtree. The path is perilous, thanks to poor visibility, lots of cliffs, and a wide variety of deadly enemies. Eventually the air will clear, however, and you'll come out near ruins. Keep following the edge of the cliffs and you'll wind up on a slope down to the Yelough Anix Tunnel.
Yelough Anix Tunnel
There's nothing in the elevator shaft down to the Yelough Anix Tunnel, so just use the lift normally and activate the Site of Grace / Summoning Pool at the bottom. Follow the narrow tunnels ahead until you come to a door that you can't open from this side. When you turn back you'll see a hole in the floor on your right. Carefully fall down.
At the bottom of this drop you'll start running into miners, near a frozen lift. Kill them, and check the body on the lift for three Freezing Greases. Among the rubble behind the lift is a Smithing Stone [8]. It's small and hard to see, but it's there.
Follow the tunnel east from the lift room. You'll enter a wider chamber with a miner on your right, and two more on both sides of the far end of the chamber. Bridging the gap in the middle of the chamber is a narrow bridge where you can find a Golden Rune [10].
Below these platforms, accessible via a ladder, are several smaller chambers where you can find several miners, as well as a Lesser Alabaster Lord. It will follow you around the room slowly, using gravity magic to pull you into striking distance. You can use the narrow passages to avoid its magic and snipe it from a distance. You can also climb the ladder and shoot it as it clambers up to attack you. The Lord will not be able to cross over to get at you, giving you ample opportunities to chip away at its health. These lower chambers contain three Smithing Stone [8]s, a Rune Arc, and an Ancient Dragon Smithing Stone.
Climb back up to the platforms. Check the northeastern exit of the chamber to find a side room containing a miner, six Explosive Stone Clumps, a Somber Smithing Stone [8], and a ladder. Go up the ladder to find a platform with two more miners and a Smithing Stone [8]. Continue up the next ladder and you'll come to a room guarded by an Onyx Lord, which fights pretty similarly to the Lesser Alabaster Lord. A body in this chamber contains a Hero's Rune [5]. Check the edges of the room for two Smithing Stone [8]s, a Somber Smithing Stone [8], and a Somber Smithing Stone [9].
West of the Onyx Lord's chamber you'll find two doors. Open the smaller door on the left and you'll create a path back to the entrance of the Yelough Anix Tunnel. The double door on the right, however...
Astel, Stars of Darkness
The counterpart of Astel, Naturalborn of the Void, this version of Astel fights identically to the other astral beast. Astel has an enormous reach, both with its beam attacks and sweeping arms, and will try to kill you with a mixture of tail swipes, shockwaves, explosive clouds, and area-of-effect fields. Here are some hints for bringing Astel, Stars of Darkness down:
- Aim for the head. Regardless of your weapon you should try to pummel Astel's funky skull. These attacks will do more damage, and lower Astel's poise at a greater rate. Stunning Astel as often as you can is highly advised.
- Try to stay in front of Astel. This is easier to accomplish with ranged weapons, but melee fighters should do the same. Not only will this allow you to aim at Astel's head, you'll see its various attacks coming. This is particularly important if Astel decides to use its shockwaves or its area of effect gravity attack.
- If the air around Astel begins to waver, run. Astel's gravity attack is about to go off, and unless you have lots of health it can OHKO you if you get caught.
- Similarly, if Astel teleports, run. Astel will either appear elsewhere on the battlefield, which is no big deal, or it will appear above you, in which case Astel will try to grab you. If you get grabbed you're going in Astel's mouth, and the chances of survival are small. Running will generally allow you to avoid this attack.
- Call in summons, but try to use ranged Spirits. Melee summons tend to get ripped apart by Astel's frequent shockwave attacks.
You'll earn 120,000 Runes and the Meteorite of Astel Sorcery for defeating Astel, Stars of Darkness. You'll also finish up in Yelough Anix Tunnel, so feel free to leave the way you came in.