Why does The Invincible take the form of an eye?
The Invincible does not take form of an eye - it contains an "eye" built into the ship.


  According to ancient Terran recordings at the ruins of Oeilvert, the Invincible was developed many times to refine it into a perfect tool of war. The final version was entrusted to Garland, who recognized its potential as a weapon. The Invincible's underside contains a large, crimson "eye" that fires a devastating energy beam and can enslave eidolons.