First discovered in the city of Sia Marhu, the luminous siro appears to be a form of flatworm, about the size of a large dog. Their flesh is a brilliant white and faintly shining from an unknown source, along with a row of blue bioluminescent patches down the back. In motion they are ghostly and elegant, moving slowly through the air under their own power.
The siro's body is even more simplistic than its natural cousins'. There is no digestive tract or even differentiated organs apart from the luminous patches and the creature's simple dark eyespots. A dissection of a siro's body reveals a uniform, soft, slightly translucent white tissue with a high water content. Although the creature's exact diet is unclear, siro nests (they appear to be social, congregating in large numbers) are usually inside abandoned buildings and often found to be full of dessicated corpses. The victims are often identified as people who have recently gone missing. One witness to a nest who managed to escape described the creatures slowly approaching at a speed that should be easy to outrun, yet feeling drawn into their dance and unable to look away. They also reported the sensation of a vast interconnected "network of thoughts" (they were unable to further elaborate) that the creatures seemed to be guarding, and that if they stayed behind they would become a part of its "weave". It was only by an immense force of will that they were able to turn and leave.
The luminous siro's reproductive processes remain likewise unknown, but smaller individuals have been sighted within the nests. It is theorised that these bud off from the parent siro due to the lack of specialised organs.