It was clear since the datamine that Necrozma is where the list of the UBs ends, since unlike all UBs, it does not have Beast Power as its ability.
What it does have instead, is "a renamed existing ability", in this case, it's basically Filter called Prism Body. Just like Solgaleo/Lunala (and mascot legends in general recently) have a renamed Clear Body and Multiscale.
The Primary Psychic type is another obvious clue for its ties to Solgaleo/Lunala.
Although where Solgaleo has the most "physical" hard secondary type of Steel, and Lunala has the least tangible ethereal secondary type of Ghost...Necrozma is pure Psychic. I look at that the same way I look at Tornadus in its group. Where it's a single type in the sense that both its secondary shared type of the group is Flying, but the elemental primary type it is representing in the group is Flying as well, which results in pure Flying. Necrozma in that sense to Solgaleo/Lunalas Steel/Ghost, is Psychic, the type clearly associated with cosmic themes.
Peeking at its movepool also unveils 2 signature moves, which again fits the mascots.
The odd part is that it has lower stats (600 rather than 680), but even that is another point towards it being the "third" legendary, as we've had them initally weaker in the last 2 generations as well (Kyurem and Zygarde), in particular with Zygarde having exactly 600 in its initially known form (although I hope NEcrozma doesn't take 2 games and the beginning of gen8 to become relevant, otherwise what the heck is the point??).
But the design itself is where I'm curious. Prominently black (whereas Solgaleo/Lunala both use white features to different degrees), as well as not having any yellow/gold details (which if I understand Solgaleos/Lunalas origin is rather crucial, and might be a separating factor between them and Necrozma).
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As for Marshadow.. it really was a red herring. It might have some relation to the group, but its type combo not even featuring Psychic, as well as including only one of the "element" types of the group (Ghost, while not Steel) point strongly away from any connection, towards "a standalone event legendary" in the same vein as Hoopa.
What it does have instead, is "a renamed existing ability", in this case, it's basically Filter called Prism Body. Just like Solgaleo/Lunala (and mascot legends in general recently) have a renamed Clear Body and Multiscale.
The Primary Psychic type is another obvious clue for its ties to Solgaleo/Lunala.
Although where Solgaleo has the most "physical" hard secondary type of Steel, and Lunala has the least tangible ethereal secondary type of Ghost...Necrozma is pure Psychic. I look at that the same way I look at Tornadus in its group. Where it's a single type in the sense that both its secondary shared type of the group is Flying, but the elemental primary type it is representing in the group is Flying as well, which results in pure Flying. Necrozma in that sense to Solgaleo/Lunalas Steel/Ghost, is Psychic, the type clearly associated with cosmic themes.
Peeking at its movepool also unveils 2 signature moves, which again fits the mascots.
The odd part is that it has lower stats (600 rather than 680), but even that is another point towards it being the "third" legendary, as we've had them initally weaker in the last 2 generations as well (Kyurem and Zygarde), in particular with Zygarde having exactly 600 in its initially known form (although I hope NEcrozma doesn't take 2 games and the beginning of gen8 to become relevant, otherwise what the heck is the point??).
But the design itself is where I'm curious. Prominently black (whereas Solgaleo/Lunala both use white features to different degrees), as well as not having any yellow/gold details (which if I understand Solgaleos/Lunalas origin is rather crucial, and might be a separating factor between them and Necrozma).
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As for Marshadow.. it really was a red herring. It might have some relation to the group, but its type combo not even featuring Psychic, as well as including only one of the "element" types of the group (Ghost, while not Steel) point strongly away from any connection, towards "a standalone event legendary" in the same vein as Hoopa.