From Sonic Retro
The Overmind is the controlling psychic intelligence of the Voxai, a group of Twilight Cage aliens encountered during Sonic Chronicles: The Dark Brotherhood. It resides on Voxai Colony Alpha, within Sector Charybdis.
Not a single entity, the Overmind was made up of three powerful, oversized Voxai: Leucosia (green), Ligaia (orange), and Riadne (blue). They appear to have merged their minds together, functioning as a single consciousness (they are repeatedly seen to finish each others' sentences). Normally, the Overmind would send telepathic messages to the citizenry and instruct them on how to use their time most efficiently each day. However, the Overmind's combined psychic powers are sufficient to take direct mental control of individual Voxai (or even Nocturnus), should they see fit.
Unlike the other tribe leaders in the Twilight Cage, the Overmind is genuinely loyal to Imperator Ix, willingly allying Sector Charybdis with the Nocturnus. During Chapter 9 of The Dark Brotherhood, it is revealed that Ix gave the Overmind two of the stolen Chaos Emeralds, which they are using to transform their benign and passive guiding of Voxai society into permanent psychic enslavement. When Sonic and his friends confronted the Overmind in Colony Alpha, the powerful Voxai attempted to control their minds: to "free them the burden of choice". Protected by the psychic barrier of a Voxai-treated "Great Emerald", Sonic resisted. Overconfident, the Overmind freed Sonic's friends for the duration of the battle.
Each of the three Overmind Voxai has 600HP, and have the standard Voxai susceptibilities to fire, electric, and earth attacks. They can all use the POW move Air Shield, and each has their own entirely unique attack not seen anywhere else in The Dark Brotherhood. Leucosia has Psi-Slow, Ligaia has Psi-Dazzle, and Riadne has Psi-Confuse.
After the triumvirate's defeat, what remained of the Overmind's power was taken up by Thebes, who became the new Overmind.