Nightflyers are members of Sluagh. They resemble huge manta rays with tentacles on their underbellies, their faces are far from human. They have wings and can fly. They don't have feet. Nightflyers can regenerate almost any body part.
Their tentacles are of three kinds. The thick ones which are like arms are suited for heavy work and capturing the prey; they are rubbery, almost like a dolphin. The thinner ones are like fingers, but hundred times more sensitive than sidhe's. And beneath those are shorter tentacles with darker tips which are in fact secondary sexual organs, can grow thinner and longer and have suction on their tips.
Royal nightflyer males have a spine inside their penis. The spine is necessary to make female nightflyer ovulate. No human female would survive it and it tears sidhe female inside causing them great pain.
Nightflyers are able to cling to walls or ceilings. They speak in slightly hissing manner.
Sholto reveals a myth among Sluagh, that nightflyers were created by goddess of the dead and god of the sky (revealed to be Mistral, who confirms the story) so they could bring both terror and pleasure. The goddess refused allow them to be their own people and in punishment she was torn to pieces and fed to them.