Siren Song is a monsters-only ability that mesmerises the player, preventing them from moving away from the singer. It can be blocked with silence (either via spell or a scroll of silence) or with sufficient magic resistance.
If a player has been mesmerized, they can break free by killing or otherwise incapacitating the caster, breaking line of sight with a scroll of fog or an uncontrolled teleport or blink (controlled teleports or blinks cannot be made away from the caster). Alternatively, a loud enough noise will break the spell -- for this, a scroll of noise, wand of lightning or the spell Lightning Bolt are sufficient.
This ability is currently only possessed by:
Prior to version 0.16, this ability could force the player to draw closer to the caster in addition to mesmerising them. That ability has since been moved to Avatar Song with the renaming of mermaids and sirens into sirens and merfolk avatars, respectively.