Siren's song is a monsters-only ability (non-spell) that robs the player of the volition (and thus ability) to move away from the singer. Emitted in the form of a haunting song, it can be blocked with silence, either via spell or a scroll of silence. When subject to the effect, the target is afflicted with the mesmerise status effect.
Other ways a siren's song can be avoided or broken are with the usual and expected means - possessing sufficient magic resistance (despite the fact it's not a spell...), killing or otherwise incapacitating the caster, and teleporting or blinking away. While other noises will travel a greater distance than line of sight, you can avoid the siren's song by simply remaining out of her LOS.
As mentioned, the effect itself has large range, as it corresponds to line of sight, and will lasts virtually indefinitely, as the siren will continue to sing her song until she longer can.
This ability is currently only possessed by:
Prior to v0.16, when the siren was a mermaid and a merfolk avatar was a siren, the ability could be used not only to prevent the player from moving away, but forcing progressive movement toward the singer. That ability is now the Avatar Song.