The Siren Song is a potent ability possessed by sirens of the merfolk species. This power is rooted in their unique voice box: a hybrid of a larynx and a syrinx at the base of the trachea. It's described as almost supernatural; able to lure all who hear it towards the songstress.
It is a melodious, angelic song of vocals, without lyrics to tell those who are enchanted and obsessed with the mermaid who sings it what to do in lyrics. Eventually, the Siren Song leads the person that hears it to hallucinate seeing that mermaid everywhere and hearing their Siren Song out in the ocean. The person will then be driven into the water to find that mermaid but eventually drown.