Tomegathericon

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.

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 Age
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 asks 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.

The Tomegathericon could be gained after gaining the Cyclone Chip and finishing the Gabomba Catacombs. When completed, the party gains the Tomegathericon.

Cultural Allusions
The Tomegathericon appears to be a reference to the Necronomicon, a fictional book within the Cthulhu Mythos. Notably, The Necronomicon included spells for summoning the Old Ones: anicent and powerful terrors. Dark Mage's Call Psynergy  is similar to this in that it also summons monsters.