The Ruins of the Ancient won’t be available to you until later on in God of War, but this Lake of Nine hidden area does contain some loot worth finding as well as being a stop for the ”Anatomy of Hope” Favor.
Once the water drops a second time, you’ll find a beach to dock on directly north of the Forgotten Caverns. Once you land on the beach, you’ll find a Mystic Gateway right in front of a stone structure you need to hop on over to enter the main area.
Once you start looking around, you might notice a Soul Devourer roaming around. Thankfully, you won’t need to engage with it as it’ll only attack if you strike first. If you’re looking to take it down, hit it and the battle will begin. Ancients can’t be hurt by normal means, so use the pillar in the center of the battlefield for protection, since you need to hit its exposed chest with an axe throw whenever it starts firing projectiles. Whenever you hit it with an axe toss, a piece of its core will fly out. Wait for it to stop shooting, then grab the piece of its body. Wait for its next attack and chuck the core at its chest. After a few of these, it’ll stagger and you can stun grab it and attack it directly.
As far as items you can find on the beach, one of the first things you’ll encounter is a treasure map called The Historian. There is also a chest in this area that contains a Niflheim Language Cipher piece.
You’ll also have to return to this beach while doing the ”Anatomy of Hope” Favor.