The structure is known as The Grand Cloister and is filled with bug warriors, like the ones typically found in Caelid. The first step is to Elden Ring Runes drop into the ground, where there will be the small river of red flowing west. Take this path until you see an old stone coffin on the edge of the waterfall. The game will prompt you to climb into the coffin. This is the way to get to the next location.

You'll be in a fresh underground lake area, with the exception that it's regular lake water, not scarlet rot, advance forward and you will discover a fight with a boss. Here you will have to fight Astel, Naturalborn of the Void which is a space scorpion-like creature. Once you have defeated the boss, there will be a door that is sealed in the back of the arena, which can be opened via an entrance to the Dark Moon Ring. Follow the route throughout the cave, until reach an elevator. Then take the elevator up.

Moonlight Altar

You will emerge on a high plateau in the Liurnia region, which is not accessible via other methods. When you are at the point where you emerge, you want to head north up the hill into Manus Celes, the Cathedral of Manus Celes. The route is straightforward but you'll want to hop on Torrent. Right as you are about entering the church, a Glintstone Dragon boss will spawn directly in front church. If you're on Torrent you can ride past the dragon inside the cathedral, where you'll find a Site of Grace. To the right is a tunnel heading beneath the cathedral. It's quite dark, which is why you'll require the aid of a lantern or torch. You'll have to Elden Ring Items for sale dive into the hole by getting onto various ledges, and then towards the downwards.