When they say "never before seen", does it mean "never before seen" to everyone in the world, or just the non-Japanese audience? If it is the latter, it might be the Dusk Mane and Dawn Wings Necrozma from the Ultra Force set.
I would think that those cards would be the cover cards for the theme decks in the Forbidden Light set, since it seems since Black and White. These sets have video game cover pokemon as theme deck cover cards:
Next Destinies
Boundaries Crossed
XY
Primal Clash
Guardians Rising
Maybe Forbidden Light?
Is it possible to have the same card as a theme deck cover card and as a promo card in a box set, when both are released roughly the same time?