At some point during the 5th century BCE, the Followers of Ares claimed the cave for their own. During the Peloponnesian War, the Spartan misthios Kassandra explored it, stealing the Followers' treasures and killing them.
Behind the scenes
In Assassin's Creed: Odyssey, Kassandra will make a comment upon entering the cave, referencing the Battle of Thermopylae where a traitor helped to sneak the Persian army around to surround the Spartan army. After exiting the cave Kassandra will make a comment (All my life I've heard stories of Leonidas. I can't believe I'm finally seeing where history was made.)