Just like its name suggest, it is about a large number of wraith swarming to your house (similar to blood moon, but instead of focusing the increase in zombies, its the wraiths (spawn rate of other creatures will increase as well, but drastically for wraiths)).
this event might occurred after destroying a shadow orb after defeating WoF (or at hardmode). later at night, instead the normal moon light, the sky will be black with only a few stars. and also, you will gain the debuff of decrease in vision as soon as the event start (night start).
as an extra, new sentences will be added to the dialogue of the NPCs during the event (for the males, they will be commenting about how the moon did not appear/how the sky is dark (example:its night alright but its darker than it has to be). for the females, they will show that they are terrified (not angry like during blood moon) (example: why do i get the feeling someone is watching me?)