The Tomegathericon is an item in Golden Sun: The Lost Age. It appears to be a black book with small red markings. By equipping the Tomegathericon, the user gains the Dark Mage class series. It is one of three items in the game that changes the user's class. It's best used slightly later in the game (relative to when it is obtained) when the player has a large amount of Djinni and also understands some of the more advanced tactics.
Being an Artifact, the Tomegathericon can be sold and later repurchased. It can be bought for 999 coins and sold for 749 coins.
In Golden Sun: the Lost AgeEdit
The Tomegathericon was originally located within the Great Gabomba in Kibombo. After being recognized as Kimbombo's Witch Doctor, Akafubu was supposed to be given the Tomegathericon. However Akafubu had rushed out of the statue. Felix and the party, who had helped and accompanied Akafubu, were the only ones left. The Great Gabomba asked Felix to relay a message to Akafubu, that he must now earn the magic. Kraden takes advantage and asks if they could earn the magic before the young Witch Doctor. The Great Gabomba allows them to compete for the knowledge, hoping the competition would serve as positive motivation for Akafubu.
After relaying this message, the Tomegathericon can be gained after obtaining the Cyclone Chip and finishing the Gabomba Catacombs. In the final chamber of the optional dungeon, the Great Gabomba presents the party with the Tomegathericon, although along with it are the instructions to not tell Akafubu that he had been beaten to the treasure, so as to not discourage him. The spirit of the Great Gabomba then rests, hoping to one day bestow the magic to its rightful bearer as well.
The Tomegathericon is referred to in the Japanese as the Necronomicon. The name originally belonged to a fictional book within the Cthulhu Mythos. Notably, the Necronomicon included spells for summoning the Old Ones: ancient and powerful terrors. Dark Mage's Call Psynergy is similar to this in that it also summons monsters.
Additionally, To Mega Therion is Greek for "The Great Beast" and is a reference to the Book of Revelation (the last canonical book of the New Testament in the Christian Bible). The fact that the book can be bought for 999 coins may also be a reference to this. (If you flip 999 upside you get 666, which is sometimes called the "Number of the Beast").
In any case, however, it is possible that the developers had set a placeholder name for the file (literally "Tome Gather Icon") and forgot to change it.