SPOILER Aldorei Valley, "darkness Into The Sky To Open Door"

the quests recently havent been very informational. You basicly get a vague instruction like "Do the thing"

so this is pretty hard.

In Aldorei there is a sign in a Cellar downstairs that says "When you let darkness into the sky, this door will open"

and then there is a door next to it. I have no idea what that means, can someone please help.

 

When it is night time it will open.

See here if you want to spoil all the hints:
Aldorei's Secret Part I Is Wrong