Found on the western edge of the Freezing Lake in the Mountaintops of the Giants, the Spiritcaller Cave is a brief but fairly unique dungeon. Literally all of the enemies here are snails and their spirit summons, and the boss fight involves defeating two different boss spirits, one after the other.
From the site of grace, head east into the dark tunnel — turning on a Lantern or equipping another light source will help here, especially with avoiding the trap just ahead. The tunnel will curve south, and you’ll enter a small chamber with a summoning pool. Continue south-southwest slowly, and stick to the wall (rather than moving through the middle of the room). You’ll soon enter a larger chamber, where there is a section of floor that will collapse if you step on it.
Make your way around the trap and continue into the tunnel to the south. You’ll be confronted with a small drop — below you are a few spectral wolves, and in the far corner is the first spiritcaller snail of the cave. Characters with long-ranged attacks can just kill the snail from the high ground or from the log, while melee characters have two choices: kill the wolves and rush the snail, or completely ignore the wolves and go straight for the snail. The wolves will be resummoned fairly quickly, but killing them does make it easier to get to the snail. Once the snail is killed, all of its summons will disappear (this is true throughout the dungeon).
Make sure you grab the x5 Arteria Leaf from the corpse next to the snail, then turn right to continue southwest into the tunnel. It will curve west, and then you’ll drop down a small distance from a ledge. After dropping down, turn left when you near the next clump of Cave Moss, and head south into the tunnel — you’ll find a Golden Rune , Hero’s Rune , and a Rune Arc at the end of the tunnel.
Go back the way you came (north) and then turn left to head west, deeper into the cave. This next tunnel will wind downhill for awhile, and then you’ll come out into a larger chamber. To the left (northeast) is the path from the trap (that you avoided at the start of the cave), so there’s no need to go that way. Instead, look down over the ledge to the southeast, and you’ll see a spiritcaller snail below you. You can jump down and kill it, or use ranged attacks if you’ve got ’em. Then drop down to where the snail was — a corpse nearby has a Golden Rune .
Snatch up the rune and continue south — start sneaking once you pass through the “doorway” in the rock. There are a number of patrolling spirit wolves here, and once you aggro one, the rest will come too (and so will a humanoid summon). Use the multitude of bushes to make your way to the southwest corner of the chamber (the far right), where you’ll find a spiritcaller snail hiding. Make sure you grab the White Reed Set from the corpse by the tree on your way to the snail!
Killing this snail will remove some — but not all — of the spirits in the chamber. Stay crouched, and from the (dead) snail, go southeast to find a Golden Rune , then continue moving southeast through the bushes until you spot the other snail across the chamber to the northeast. Go as far as you can in these bushes, then look northeast to find the other snail. You can rush it and kill it quickly, or sneak all the way back to the start of this room and then sneak down the left side of the room to reach this snail without being spotted by spirits.
With both snails dispatched, all the spirits will be gone, and you can head south into the tunnel that leads all the way to the golden fog gate — when you’re ready, head on through to face the Spirit Snail and his summons.