Night-haunts are winged creatures that eat the bodies of dead Faeries for the memories they contain. When someone fae dies, the night-haunts rapidly appear to take the body away. They leave an exact (humanized) replica behind. The Faeries made this arrangement with the night-haunts long ago. Without them, the bodies would remain forever, because faeries do not age or decay.
In certain circumstances, night-haunts can become Fetches, warning of an impending death. It is revealed by the Luidaeg that the night-haunts were a scourge among the Fae, eating both living and dead, until Oberon bound them to eat only the dead. 
Night-haunts take the face of the body they've eaten most recently.
There are multiple flocks of night-haunts, scattered all over the world. They usually live in groups of about thirty to sixty, depending. They have a higher than usual birth rate for fae, because they also have a higher than usual death rate; a night-haunt who doesn’t manage to taste a death before their last one runs out will literally starve and disappear.
- ↑ Rosemary and Rue, ch. 26
- ↑ 2.0 2.1 Rosemary and Rue, Ch. 4
- ↑ http://seananmcguire.tumblr.com/post/98622774765/a-nighthaunt-question-from-rereading-the-books