This section of the underground Ainsel River leads to the city of Nokstella, and is guarded by claymen and ants. You’ll also run into a strange, but likely familiar foe with a propensity for rock flinging. You won’t be able to reach this area until you’ve done Ranni’s dirty work in Nokron, but the treasure in Nokstella is well worth the effort for anyone who likes using spirit summons, and the Eternal City is another key step in Ranni’s questline.
To access this area, you’ll first need to visit Nokron for the Fingerslayer Blade, and then venture to the Divine Tower of Liurniaand collect the Cursemark of Death. Once you collect the cursemark, Renna’s Rise (the northeastern tower in the Three Sisters) will open. At the top of the tower is a waygate that will take you to Ainsel River (Main).
If you haven’t yet done Ainsel River (South), it’s recommended that you do so now — you’ll get a map for the whole area, but more importantly, you’ll get a chance to kill an enemy that can make it hard to grab some items when you’re in Ainsel River (Main).
After going through the waygate in Renna’s Rise, you’ll find yourself transported to Ainsel River (Main). Directly ahead of you is a coffin with the Miniature Ranni Doll inside. Grab it and then follow the river south for just a little ways to find a site of grace on your left. Rest there and use the “Talk to miniature Ranni” option at the bottom of the menu. Do this three times — on the third attempt Ranni will finally speak with you. She’ll task you with defeating the Baleful Shadow, and then you can get up and continue following the river south (those of you who like taking screenshots may want to head north to the waterfalls first).
Once you pass under some rocks that hang over the river, use target-lock to find a clayman on your left, and then again on your right. Just a little further south, you’ll enter the Uhl Palace Ruins, where one of those weird space ants will start flinging rocks at you (these guys seriously need a new trick).
Sprint south to get in cover behind the ruined wall. In the water to the south-southeast of your cover is a corpse with a Golden Rune , which you can safely grab without being attacked. From the rune, go southwest to the next bit of cover — summon your favorite spirit while behind cover. From here, you can actually charge right at the floating freaky rock flinger if you’re a melee character (ranged characters can just snipe it with spells and then duck back into cover). When you run at it, it will start to cast its spell, but just run past it and then in a circle around it — most of the rocks should miss, and you can start smacking your floating foe.
You’ll need to use jumping attacks to make sure every attack lands. It does have a bite attack, but if you’re standing directly under it and moving forward it should miss. It will drop a Somber Smithing Stone  when it dies. At this point, a couple of claymen will probably have gotten pretty close to you (if they haven’t started attacking you or your summons already). Take them out and then look southwest — the path through the torch-lit columns is the way forward, but we’ve got a few things to collect first.
Instead, go southeast and you should soon hit the rock wall with some claymen near it. If you turn right once close to the wall, you’ll see a couple of Ghost Gloveworts. Grab them and then head northeast — against the ruined wall there is a corpse with a Golden Rune .
Continue east to the end of the wall and turn left to head north — on your left near the end of the wall is another corpse with a Smithing Stone , and then further north are a few more Ghost Gloveworts.
Now go northeast from the glovewort, and you should see some dark palace ruins with a clayman in the corner — the corpse in front of him has x3 Human Bone Shard. Head back out into the river and follow it south, where some flowers guard a Stonesword Key. Run in and grab it before continuing south to where the river terminates in a waterfall. Hidden behind the tree on the far right end of the waterfall are the Clayman Ashes, guarded by two claymen.Grab the ashes and return to the where you slayed the big rock-flinger by going west-northwest.
Head southwest through the torch-lit pillars. You’ll find a few more foes here, and in the far right corner of the palace ruins is a Ghost Glovewort . Continue following the river southwest as it enters a narrow tunnel — soon you’ll find a corpse on the left with some Stimulating Boluses, and two ants on the ceiling nearby. You can fight them one at a time if you don’t rush in — deal with them and continue following the river.
You’ll enter an ant feeding ground, with two ants eating on corpse piles on the left side of the room. Try to pull them one at a time with projectiles, because further to the southwest behind them is a shield ant — this guy can’t be damaged from the front, so circle around past its head to damage it.
Once all three ants are part of the corpse pile, loot the two humanoid corpses to find an impressive x8 Golden Rune , as well as x2 Shield Grease. Then go south into the hole the shield ant was guarding and follow the tunnel until you reach a cliff that overlooks part of Ainsel River (South). Sneak up on and kill the guy looking out over the river, then turn right and kill his buddy too.
Next you’ll have to drop down onto the rocks to the northwest, continue up the narrow path, and drop once more to the path below (to the west)
Follow this path around the rocks until you pass a pillar on your left. At this point, you can drop down to the left (southwest) onto the stone wall, and make your way to the chest to find the Wing of Astel.
Now go back onto the path you dropped from, and jump up the rocks heading northeast. Then go east-northeast to the Somber Smithing Stone , and you can jump down to the east and make your way back to the corpse pile room.
Take a left to follow the river northwest, where you’ll find an aqueduct-like structure. Grab the Ghost Glovewort  in the water, then head up the stairs through the tall doorway to find Nokstella, Eternal City — drop down the two platforms and grab the city’s site of grace.