The Long Night has many names around the world, some call it the Devil's night, some say its the God's looking away, regardless of it's name or why it occurs, it is a time of mysticism, myth and wicked events.
Every few generations, sometimes 2, sometimes 7, sometimes more; the sun fails to bring it's light to earth.
The Moon Rises, sets, Rises and Sets before a single ray of daylight is seen.
It is believed that during this time, the realm of shadow and darkness comes and touches our own, and during this time, wicked things migrate from that realm to ours.
Things of Legend, of myth; things that kill, that kidnap, things that desecrate what most hold dear.

In your Immortal state, if you have been around long enough, You have more than likely encountered some event such as this, maybe even a creature of the Night or two. There is no way to tell or determine when the next Night will occur, or what may even come of it

Creatures of the night can only come into this world at this time. however, often they stay in this world after it recedes, sometimes for years.
Creatures appear in areas akin to their own habitat; E.G. fey appear in forests, undead and zombies in graveyards, trolls in caves, etc.