The Altar of the Emeralds, also known as the shrine of the Master Emerald or the Pillar (ピラー Pirā?) in early Japanese materials, is a place on Angel Island where the Master Emerald is kept and guarded. It is a mythical ruin surrounded by seven columns which are designed to hold the other Chaos Emeralds. The altar made its formal debut in Sonic Adventure, but it was also mentioned in the Japanese manuals of Sonic the Hedgehog 3 and Chaotix. In Sonic & Knuckles and Sonic Mania, a similar structure is seen in Hidden Palace Zone, where the Master Emerald teleported to safety after the Death Egg crash-landed on the island.
A reconstruction of the altar appeared within the Space Colony ARK in Sonic Adventure 2, having been designed by Professor Gerald Robotnik. The said altar contains an eerie apparatus above it that functions akin to a beating heart.
Sonic Adventure shows the history between the Knuckles Clan and the eight Emeralds through the use of flashbacks. The Emeralds were housed and guarded by Chaos, a Chao mutated into a water spirit who is considered their first guardian. He guarded the eight Emeralds for an unknown (albeit presumably long) period of time, and the power of the Emeralds was passed through oral tradition.
When the nearby tribe attacked the altar in order to fuel their expansion of conquest, Chaos was enraged and used the seven lesser Emeralds to wipe out most of the tribe. He was then challenged by his gentle friend Tikal and sealed within the large Master Emerald along with Tikal herself. The tribe's territory miraculously lifted into the sky through the sheer power of the same huge Emerald. The Emeralds became known as Chaos Emeralds, and Chaos himself became known as the God of Destruction. The remaining echidnas swore to protect the Emerald so that a tragedy such as this would never happen again. The only living descendant of this lost civilization is Knuckles the Echidna, who is determined to guard the Master Emerald and protect Angel Island at all costs.
In the anime series Sonic X, the Altar of the Emeralds had a backstory similar to the one from games. It was once part of the Mystic Ruins on Sonic's homeworld where the Chaos Emerald and the Master Emerald resided, and was once inhabited by the Chao and Chaos.
In the Archie Comics, the Brotherhood of Guardians is a fictional organization of anthropomorphic, humanoid echidnas in the Sonic the Hedgehog series. They are a group of Echidnas who protect Mobius from a secret facility known as Haven and the Master Emerald. They are also Knuckles the Echidna's predecessors and family in the title of Guardian. The Brotherhood's existence is secret to all but a few individuals, including their arch-enemies, the Dark Legion. The Emerald Altar was originally in a small cave but then became a small island off the coast of Angel Island during the battle against Super Sonic and the second Enerjak and now has a small bridge connecting them.
- The Altar's appearance and location have changed several times during the course of the Sonic the Hedgehog game series, suggesting that more than one shrine may exist on Angel Island.