So, if i am correct "Zygarde Complete Forme" ends up getting a useless Ability, unless Power Construct has another effect which is useful on Complete Forme.
Have you seen that thing's base HP? If the rest of its stats are anything like that, it doesn't need that much of a useful ability.
Actually, after doing some more research on Zygarde, it may end up being a Legendary that you can't kill unless you used a One-Hit KO move. Why? Well, now that I think of it, and most likely this is a gigantic stretch, but perhaps Zygarde's Power Construct ability may also work backwards. Like, say when Zygarde Complete Forme goes to 50% of its total health, perhaps it devolves back to Zygarde 50% Forme, but when the latter goes into low HP itself, it returns back to Zygarde Complete, which in turn, as we already saw with Zygarde 10% Forme, its HP recovers/increases back to the value of Zygarde Complete Forme's base HP state. This, in effect, establishes some sort of endless loop of regression and evolution, never ending. This may explain why the Complete Forme has the Power Construct ability.
If this is true, Zygarde's endless loop theme may refer to the legendary
Ouroboros serpent which represents infinity and immortality, not to say, it also happens the Ouroboros has a connection with alchemy as well. It goes even further with having associations to the
World Serpent Jörmungandr which Zygarde 50% Forme is based on, as well having connections to the cycle of life and death, which in return was the theme with the XY Legendaries, etc. In essence, Zygarde Complete/Perfect Forme would be the first PKMN to be
immortal by some degree.
I mean, imagine having a PKMN that literally can't be fainted because, via its Power Construct ability, its constantly restoring back its HP back to the base HP stat it has in Complete Forme? Zygarde would become pretty much semi immortal unless it got hit by a super effective attack that manages to deplete all its HP in one single turn before it can actually use its ability, which is highly unlikely as Zygarde is sure going to have some high defensive power/stats to back up its high HP stat, not to say it appears to be a pretty bulky PKMN in design.
Of course, its a pretty big stretch, but it would make sense as to why it has a supposedly "
useless" Ability at first glance.
