Arcadia

Arcadia is the city-sized flying battleship that Luxion has spent millennia maintaining underground. It carries his core, a fleet of unmanned support craft, hundreds of stored Old Humanity Armors, and enough firepower to vaporise any modern military force in the sky.

When Leon registers as Master, Arcadia becomes — quietly and inconveniently — his. He hides it for as long as he can, but every major arc eventually drags it into the open: the Fanoss war, the Alzer crisis, the late-arc imperial campaigns all end with Arcadia in the sky rewriting whose problem is worse.

It is also the show's most pointed image of what Old Humanity actually was: a civilisation that built warships the size of cities and then lost the war anyway, leaving Leon to inherit the apology.