The Whispering Plague
The merfolk possessed stones that could hold powerful magic and the Sea Queen believed it was these that would bridge the gap between her world and the one of her lover. The Warrior and the Sea Queen believed their love was strong enough to survive the consequences for breaking the sacred rules of their people. So the Sea Queen used a Mermaid Gem to turn her lover into a merfolk.
The transformation was successful but unleashed a terrible curse that robbed the Queen of her song. The curse became known as the Whispering Plague and could be transferred by a simple scratch, spreading quickly to the Sea Queen's people.
The Song of a merfolk is part of who they are, their soul and their collective memory. Without their song these merfolk became something else, something twisted and leaving only darkness.
Those afflicted were exiled, likely as a preventative measure to keep it from spreading. This new branch of merfolk became known as the Sirens. When they found their voices once more, theirs was now a song of sorrow — the Siren Song — a grief for all that they had lost.