What is the original name of Final Fantasy XV?

The original name of Final Fantasy XV was Final Fantasy Versus XIII. During the early stages of development, the game was planned as a spin-off to the main Final Fantasy series, and it had a darker and more mature theme compared to the traditional Final Fantasy games. However, as the development progressed and the project underwent significant changes, including a shift in storyline and gameplay mechanics, the game was rebranded as Final Fantasy XV to reflect its evolution into a full-fledged mainline installment in the series.