From the site of grace, go past the summoning pool, where you’ll find the start of a tunnel blocked by an illusory wall. Roll into it to remove it, then head through to enter the dungeon proper.
You’ll enter a large chamber — there’s an enemy sitting to your right that you should take out first. Then continue down the path to the south-southwest, and turn left at the fork to head up the wooden stairs that lead to a wooden walkway. Around the corner, you’ll find a corpse draped over the edge of the walkway with a Golden Rune . Go back the way you came, and this time continue down the path that leads southwest to the low ground.
There is a vulgar militiaman waiting on the rocks to your left at you reach the low ground — take him out and then look west to see a chest that contains the Smithing-Stone Miner’s Bell Bearing . Behind the chest is an illusory wall — roll into it to reveal a tunnel that leads to the elevator that’ll take you deeper into the dungeon.
We don’t want to take the elevator yet though, so turn around and head towards the miners on the eastern side of the chamber. One stands watching over the others, so take him out first, then kill all three miners on the low ground and grab the Cracked Crystal they’re mining. Then turn north-northwest and jump back up the rocks. You’ll see a miner sitting in a dark corner — behind him is a Smithing Stone .
Now head to the tunnel behind the chest and continue into the room with the elevator. Instead of activating the elevator, jump onto the far end of the left-hand beam that supports it, then drop down onto the root below.
After dropping onto the root, turn around and go southwest until you’re above the Smithing Stone , then drop down onto it. Then walk to the edge of the ledge (north) and turn left, then walk to the west-southwest edge of the ledge. You should see a ledge below you to the west — if you fall off in that direction while walking full speed and hugging the wall, you should land safely on the next ledge. Drop once more to the north, and then down onto the ground.
Proceed south through the roots, and roll into the dead-end wall to dissipate yet another illusory wall. Keep going south and you’ll find a corpse with a Golden Rune  — just behind the corpse is one more illusory wall. In the next room, walk up onto the large root that spans the room, and follow it southeast until you can drop down to the right onto the next root. Follow this root west until you can drop down to the left (south) onto the wooden platform below, but be careful — three vulgar militiamen wait for you on the platform.
Once they’re all dispatched (or they fall off the platform), head south into the tunnel the platform connects to — at the end, you’ll find another militaman and a corpse with a Stonesword Key. Go back the way you came, and walk out onto the root that’s level with the wooden platform. There’s a corpse with a Gold-Pickled Fowl Foot, and another couple vulgar militamen on the platform at the other end of the root. Also note the statue below the platform:
Once you’ve grabbed the item and killed both enemies, drop from the wooden platform to the ground below and stand in front of the statue. An iron maiden will approach from the west — stay near the statue and let her come to you. The goal is to have her do her arm-swinging attack, which will break open the statue and let you get the Smithing Stone  inside. You can choose to summon spirit ashes here, but it can make it harder to get the maiden where you want her.
Once you’ve gotten her to break the statue, feel free to use summons, or just grab the items around here and escape rather than trying to kill the maiden. Loot the three corpses to find x3 Furlcalling Finger Remedy, a Rune Arc, and a (disappointing) Golden Rune . In the northwest corner of the room (where the maiden was waiting) you can find a Somber Smithing Stone  embedded in the wall.
Now climb the ladder in the middle-ish of the room, and use the roots to cross the room again, until you’re on the platform above the (hopefully broken) statue. Continue east into the next room, where you can turn right (south) to drop down between the platform and the wall. There are hard-to-spot enemies down here that are clumps of stone (similar to the worm-like guys that hang around Evergaols) that will explode when you get close. Run/roll away from them when their eye starts to glow to avoid taking damage.
Go to the western wall to find a Smithing Stone , then head southeast to under the stairs, where a corpse holds x5 Grace Mimic (but watch out for the exploding enemy that waits nearby). Then go up the wooden stairs, past the explodey-guys and up the next set of stairs to grab the x2 Old Fang. Head back down the stairs to the platform and turn left to look northeast, where you’ll find x3 Large Glintstone Scrap down a short dark tunnel. Then go back to the wooden platform and go down the stairs, and continue south-southeast into a dark corner. Another exploding foe is here, but so is an illusory wall on the right side of the dark corner.
In the next cavern, walk out onto the first root, then continue forward until you can safely drop east onto the root below. Then continue east to the wall, then drop to the south to the ledge, and then down to the west onto the Smithing Stone . Walk all the way to the west of this ledge and drop onto the root below, then head northeast to the far wall. From here, jump to the north-northwest to reach x4 Lightning Grease.
Jump back onto the root, then follow it downward to the southwest, and jump off to the northwest onto the ledge below. Then drop down to the ground near the corpse, but avoid the exploding enemies before you grab the Golden Rune . From here, you can head south through the tunnel, through the chamber beyond, and into the boss arena to face the Onyx Lord.