>>56205439The game isn’t clear on what they are. Sycamore speculates that they’re the batteries for the ultimate weapon, and that their energy created mega evolution by irradiating regular stones. AZ’s weapon reaches space and emits a burst of light before falling back to earth, so it implicates ORAS’s mega evo meteors as having been irradiated from that energy too.
Like in other games directed by Masuda though, it’s not clear on whether they’re actually beings ruling over giving and taking life, or if those are just the legends written about them. There’s an old man in Anistar who speculates Xerneas’s powers may have been made up by people during the war to brighten their bleak outlook, while Yveltal’s were made up to justify the senseless violence at the time. It would track with other statements Masuda has made about legendaries- that Kyogre and Groudon didn’t actually create the planet, that Arceus isn’t actually a god, and so on. They really may have just been exceptional energy reserves for AZ’s weapon- after all, even in Y version Yveltal’s powers are used for immortality and in X, Xerneas’s for destruction.
