Ghosts will spawn during nighttime.
Advanced options: Settings -> All Settings -> Mods -> mobs_ghost_redo
According to ancient Romans' folklore they can't be hurt, save using metal weapons.
According to popular folklore, silver weapons will be more effective against them.
This mob has been ported from the Creatures MOB-Engine.
Floaty
A nuisance that moans in your general direction, love it :)