If it wasn't clear that Necrozma would be playing a much larger role in the plot of Pokemon Ultra Sun & Ultra Moon by the images (presumably) of both Solgaleo and Lunala fused with it - or even as far back as in Sun & Moon where it received relatively no storyline whatsoever - it is almost a sure thing after the most recent leaks of images. As you can see in the picture above, a player looks to encounter (perhaps even battle) Necrozma at what looks like the Altar of Sunne / Altar of Moone.
Could a player face both it and the cover legends apart of Pokemon Ultra Sun & Ultra Moon? Perhaps with the two even fused as one? Only time will tell...