The Ancient Forge, otherwise known as the Forge of Hephaistos, was an Isu vault located on the Greek island of Andros. The Ancient Forge was the site of the creation of a number of Precursor relics and Pieces of Eden, including the Spear of Leonidas.
Locked behind a triangular door, the Ancient Forge featured a corridor with a high ceiling, at the end of which was a circular area flanked by at least four unique molds; one for the Spear of Leonidas, one for what the Spartan misthios Kassandra speculated to be the Sword of Damokles, and two more for what appeared to be the Minotaur's axe and Brontes' bludgeon, or weapons sharing their shapes.
Despite the ruined state of the Forge by the time of the Peloponnesian War, it was still able to function enough to improve a weapon to the point of repairing it, as Kassandra discovered with the Spear of Leonidas. With each visit to the Forge, the broken haft of the spear was gradually turned into a polished pommel. 
The vault was discovered by the Greek historian Herodotos during his travels prior to 431 BCE. However, as it could only be opened by Isu relics, Herodotos was unable to enter the vault and learn its secrets. The Chimera, a member of the Silver Vein branch of the Cult of Kosmos, also discovered the location of the vault, demanding men and resources to aid her expedition to the vault.
After meeting Kassandra, Herodotos accompanied her to the island. Kassandra sought out the entrance, finding remains of people who had attempted to enter the structure. She used her heirloom, the Spear of Leonidas, to open the vault. Once inside, Kassandra used one of the molds to upgrade her spear with the Artifact Fragments she had collected from Cult members.