This specifically is an easter egg ending in Chrono Trigger, requiring you to have beaten the game once before to access it.
Chrono Trigger allows you to warp to the final boss and try to beat them before you would reach them through the plot, from very early on in the game. A lot of the special endings require you to do that and win at various points in the story, which isn't possible for most of the game unless you carry over stuff with new game plus.
I believe this ending requires you to start new game plus and then take the warp to fight the final boss as soon as you possibly can.
Don't quote me on that though, the game has something like 13 endings and I canct keep them all straight.
