The Overmind is the controlling intelligence of the Voxai race, the authority that sends telepathic messages to the citizenry and instructs them on what to do each day. The members of the group appear to have merged their minds together, functioning as a single entity (as they are repeatedly seen to finish each others' sentences). Unlike the other tribe leaders in the Twilight Cage, the Overmind is genuinely loyal to Imperator Ix, willingly allying Sector Charybdis with the Nocturnus.
The Overmind's combined psychic powers are sufficient to take direct mental control of individual Voxai (or even Nocturnus), should they see fit. When Sonic and his friends confronted the Overmind in Colony Alpha, the powerful Voxai attempted to control their minds in order 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, but this ultimately lead to their destruction; Leucosia, Ligaia, and Riadne were defeated.
What remained of the Overmind's power was harnessed by Thebes, who became the new Overmind.
The three members of the Overmind (Leucosia, Ligaia and Riadne) are named after some of the sirens of Greek mythology. Like the Overmind, the sirens could also take control of people, although they did it with their enchanting songs rather than through telepathy.
Another named siren in some sources is Thelxiepeia, who is presumably the namesake of another Voxai, Thelxe.
When first confronting the Overmind, Sonic isn't affected by the Overmind's power. If the player has Cream the Rabbit, Big the Cat, or Omega on their team they won't be affected either. Any other party member will be affected, uttering a brainwashed line, which is actually the characters speaking their minds. Shade speaks her mind whether she is in the party or not. However, Knuckles doesn't speak what's on his mind and Rouge will say she can't resist the Overmind.
Shade feels some regret about betraying Ix.
Tails feels uneasy and doesn't want to fight.
Amy will say that Sonic never pays attention to her. (this is her line whether you've been nice to her or not)
Shadow will say that he's tired of listening to Sonic
The reason for Sonic, Cream, Big and Omega not being affected by their power may be because:
Sonic possesses a Great Emerald or his strong will.
Cream being too innocent.
Big already has a simple mind, though the Overmind's power gives him a headache.
Omega being a robot (robots not having minds to control)