I actually have an interesting headcanon regarding the sirens, and that is that there was an entire race of sirens once and they all had the same abilities as those that we associate Adagio, Aria, and Sonata with, but they used their abilities for a different purpose. Twilight only explained that they had the power to charm ponies with their music. She didn't say whether the only purpose of this power was only to stir up trouble to feed off of negative energy. Rather, I think that the sirens showed up when trouble was already in the air, fed off of the negative energy, and used their powers to try to end the trouble by singing. Sure, the siren race never really got that much stronger since they only used their powers to stop fights and cancel out negative energy flows, but when Adagio, Aria, and Sonata came around, they were the only ones who saw their powers as conduits for what they could really do, and thus began their routine of forcing ponies to fight and argue, creating an endless supply of negative energy for them to feed on and increase their powers. Fearing what ponykind would think of them because of Adagio, Aria, and Sonata's acts, the entire siren race fled and disappeared.





In this picture, it seems that if they ever were to return to Equestria, not only would their powers still be forever lost, but they might see the errors of their mistakes and the terrible outcome of it regarding where their kind may be, that is if they can still be found.

