Siren sightings were a rare phenomenon even in the most dangerous nightmares. These Ethereals lived in the darkest and coldest regions of the Nightscape, and they weren't known for hunting prey far from their territory.
The recent encounter with the Devourer and now, the appearance of a flock of Sirens suggested that someone had opened a Nightmare Portal or Gate nearby. Xander had no doubt in his mind that this was the work of the cultists, a deliberate ploy to prevent anyone from following in their tracks while creating as much chaos as possible.
Since Sirens had weak eyesight and relied entirely on their sense of hearing and unique echolocation abilities, they liked luring their prey. And the most effective way of attracting the attention of unsuspecting Dreamwalkers and wandering Ethereals was by wailing in feminine voices.