I'm really enjoying everyone's theories, but I couldn't help but think of
The Seed and The Sickness when reading these lines (further helped by Toph and Cariane investigating the spread of Nature's influence after TSatS):
Quote:
“The Green One laughed, the Ice did too,” the heads chanted in unison.
Quote:
The day Mother swept us into the goo!”
Quote:
“MOTHER RAGED, MOTHER CRIED,” the heads intoned as they spun their opponent around in a circle. “She didn’t know WE SURVIVED!” the dragon’s heads roared, as their spin picked up speed.
In The Seed and The Sickness, there's mention of an Armistice following a Great War between Plague and Nature:
Quote:
"This ceremony serves as recognition of the great war between the Plaguebringer and the Gladekeeper and observes our continued Armistice. The chaos and death from that dark era must never again come to pass."
For those who aren't aware, an armistice may be a partial or temporary cessation of hostilities - called a local armistice or truce - or it can mean a general armistice (i.e., a total cessation of all hostilities). However, all parties involved have to agree to the armistice for it to take effect. What if one of the conditions of the Armistice between Plague and Nature was the demilitarization of Plague's forces? I.e. the Plaguebringer had to get rid of her "weapons", the Aberrations - AKA her
own children. That may be why Plaguebringer swept the Aberrations into the Wyrmwound despite clearly being unhappy about it, and why the Aberrations are so obsessed with getting stronger beyond the typical Plague philosophy (since if they were stronger, Plaguebringer might have been able to win the war outright instead of accepting the armistice). The Gladekeeper may or may not have had to do the same, depending on who was ahead at the time the terms of the Armistice were created - who knows, maybe we'll see a bunch of Nature Ancients crawl out from under the Behemoth or something.
As for why the Icewarden would have "laughed" at the destruction of the Aberrations, it could easily have been for reasons as simple as seeing them as a threat to his territory and Gaolers (as the Banescales were) and being pleased that they were now one less thing for him to worry about.
Also, on an unrelated meta-lore note, I love how the Aberrations are a reference to the original concept of Mirrors, with the two heads/two tails "mirror" design that gave Mirrors their name (though only the 4 eyes thing made it into the modern Mirror design). Both in-universe and out-of-universe, Aberrations
are proto-Mirrors!