Ancient Ruins
Ancient Ruins is a place found in Devil's Spine.
Its dock appears to be made of stone and several statutes of hooded figures stand by, and appear to glow when you’re lacking in sleep. An Ancient Lighthouse and Cave are visitable here.
The Cave
Visiting the cave it is noted to be dimly lit, with paintings on both walls on ether side of you. Checking the left wall states that “These paintings depict the birth of a civilization. A great beacon of light is shown repelling a tentacled figure emerging from the sea.”
Checking the right wall instead states that “these paintings depict the fall of a civilization. A volcanic eruption tears through a tall lighthouse. Monstrous shapes devour people as buildings turn to ruin.”
This appears to be the story of the civilization that once lived here in The Devil’s Spine. A civilization protected by light that is destroyed when an eruption tears down their lighthouse.
Talking to the Travelling Merchant and asking them “what was this place?” Also sheds some light on the subject. They say that “Some kind of ancient civilization must’ve lived here. Judging by the state of the buildings, things didn’t go to well for them. I can’t believe they built this all around an active volcano. So the volcano must’ve become active some time after they founded their city. I’d guess that the volcano caused them to leave - or wiped them out, one way or another. All in all, it’s a pretty inhospitable place. I’m not really sure why I come here.”
The Ancient Lighthouse
The Ancient Lighthouse is a dilapidated stone lighthouse that is one of the few tallest still standing structures on the Devil’s Spine.
When it’s first found it is locked with a door, requiring the player to Complete the “Stone Tablets” quest, and use the “Fused Tablet” to unlock the door of the Ancient Lighthouse. Inside the player will find that “the stairway to the top had collapsed - this is as far as you’re going to get.” In terms of exploring the building. However you will also be rewarded for opeaning the door by finding the “Flame of The Sky” light as well as two goblets, a Citrine Ring, and a Ruby Ring.
Once you have collected all these objects any further visits to the lighthouse will lead to you to observe it’s crumbled state and then promptly leave.