I kinda find it particularly interesting the outfits that Dazzlings wear: this is not some everyday attire, this (headcannon ahoooya) looks like some sort of robe, probably ceremonial. Going further, I presume that after being driven to the shore, finding a new home and family, Dazzlings finally felt themselves recovered enough to make a big return to the sea, starting with challenging someone from the hierarchy they knew while they still were around. Which went swimmingly (in this case like a lead balloon).

The reason for that is that siren magic is highly specialized -- unlike unicorn one it is not taught anywhere, every siren hones her mastery by herself, through battles and victories and defeats, occasionally (although rarely does one survive that, why were the Dazzlings spared? read on...). Thus, being thrown out of the loop is tantamount to defeat -- the skills that are needed to succeed in battle against one of their own are hard to gain, hard to sustain and very easy to loose. And our trio here basically lived on a vacation -- they never actually even had to hunt their own food (sans several occasions).

But they did use their magic. On multiple occasions, while helping and comforting Sunset, while teaching Snowshine, and on other occasions Adagio used her powers to bring some extra power to her voice. Other two did it too, less commonly however. They have veered into the new territory, exploring the new utilities of their powers -- something no siren had done before, being locked in the cycle of "kill or be killed". Little did it help them in the direct confrontation, these newfound abilities will play an even bigger role in the events to come.

Now, why did the challenger left them alive? Well, with their gems shattered, sirens are no longer at the top of the food chain in the ocean. There are a lot of predators there, the sirens are some of the swiftest and deadliest. Without magic, however, they can fall prey to other ones. And this is one of the cruelest things one siren can do to the other.

Our trio, however, had another choice: flee to the shore and try to retain the anthro-form. This way, at least, they would be safe. Badly beaten and humiliated, but safe. Also no sensible siren would even attempt to do that. But then again, no sensible siren would even attempt to sleep in the close proximity to the other members of her species, so we know Dazzlings aren't big on sensibility already xD

I kinda have a few ideas about the pendant, but that is a story for another time.

P.S. That "Dazzlings vs a yet unnamed enemy" would not mark the first occasion the sirens banded together for combat. Indeed, this had happened before. But they also didn't try to murder one another after defeat (to draw the attention of the predators to the one bleeding the worst) and actually tried to support and fight back cooperatively while making their way to the shore.

Which is a first in recorded history of sirenkind.

...

Dunno how many headcannons did I broke, but don't care, had fun. All the best to Fox & Liz.