In the world of the Faerie Apocalypse - Addergoole, Doomsday, etc. - there is something that is colloquially called The Blindness of the Gods, and something called a Mask.
Actually, I have to back up a step.
There are, in Fae Apoc, three groups of human(oid)s:
* Those who are fae, who have Changed and have magic: Fae, ellehemaei
* Those who are 100% human, or near enough as to make no difference: Humans
* Those who have fae blood but are not fae, have not Changed, and often can only access a little bit of magic, if any: Faded
Like many things, this is actually a continuum, but for our purposes, it’s close enough.
A Mask is something a Fae purposefully chooses to put up. For most fae, it covers their Changed appearance - whatever part of them isn’t human-looking anymore (wings, horns, tails, animal features, elven ears, scales, etc.; my Pinterest board Addergoole Changes has a nice broad selection) and makes them look more or less as if they were human.
But that’s only important for other fae and for Faded, because of the Blindness of the Gods. For humans, it is as if all fae are always Masked; they all look human all the time.
Except on Hallowe’en.
On All Hallows Eve, the spooks and the creeps are out in force, and any human can see an un-Masked Fae for that they are. The borders between the worlds - between Earth and Ellehem, where the Ellehemaei came from - are thinner that day, some say, and some say that the magic of impressions is just that strong. Some say it was an ancient spell that broke through just a tiny bit of the Blindness of the Gods.
The truth isn’t known, but on Hallowe’en, fae like to stomp the streets in their true visages. Some do it for a lark. And others, darker fae, like to spend the night of Hallowe’en reminding humanity that monsters really do walk the same streets as them.
This all changes with the apocalypse, of course - but then again, everything changes with the apocalypse, everything except the Words of Magic themselves...