The Siren's Song

Distant first, our party heard it echo across the cliffs. The unmistakable and mournful tune of a wounded animal. We all felt it, that faint feeling of desire. It was how the Song worked its magic. It offers you a thousand contradictory comforts at once, peace through violence, freedom through slavery, satiation through eternal hunger. A few stopped in our tracks to listen, those who knew better pressed onwards. I clutched my pendant tighter, a symbol of the faith that has served me this far, and pushed onwards.

— Account of a Cieran Traveller

Manifestation

The Siren's Song is perhaps the most dangerous form of magic The Winter Court have at their disposal. Regularly throughout the Kingdoms, one may hear the alien and mournful dirge of the Siren in the distance, driving men towards madness and Thralldom. The mind controlling effects of the Siren is a constant threat to the denizens of the Kingdoms, though it primarily affects those who remain outside the radius of a Beacon for too long or those within a Beacon's influence that either willingly embraces the Song, fail to develop a sufficient coping mechanism, or give the Song purchase through being overtaken by some negative emotion the Unsidhe can exploit. Anger, fear, jealousy, and particularly feelings of grief and depression allow the Song to find purchase in the minds of mortals easier.
One observed inconsistency in the Song is its tendency to occasionally grow and encompass instruments other than the "voice" of the Siren. This has been correlated to specific Unsidhe Fae playing instruments to bolster the effects of the Song, strengthening their control over their control and augmenting the mental assault against those who aren't. A wide degree of instruments have been recorded, though not all have been linked to a documented or slain Fae. Multiple fae have been observed playing the same instruments. Instruments recovered from defeated Fae have not displayed inherent magical properties, though for some reason tend to be constructed from the grisly remains of their victims.
Each culture within the Kingdoms has developed their own ways to resist the effects of the Siren and prevent themselves from falling to Thralldom. The Cieran put faith in their dogmas, living rigid, orderly, and community-focused lifestyles to ensure their willpower is sufficiently fortified. Krels take the more basic approach of simply brute forcing their way through, relying on sheer willpower to overcome the lure of the Siren, with mixed results. Finally, the Mordova resist the Siren through detachment from their surroundings and achieving inner peace. The Siren wholly incapable of affecting those who are content with their lives and their surroundings, so most denizens of the Kingdoms strive to support one another and ensure everyone can live a life they're content with.

Localization

The Siren's Song affects the entire continent of Belia, growing more potent the further north one travels. Locations within the proximity of a Beacon aren't as affected, and locales located south or otherwise behind another Beacon or set of Beacons are impacted less. It should also be noted that the effects of the Song and its range waxes in the days approaching the winter solstice, and wanes in the days approaching the summer solstice, reaching its peak and trough on each of those days.