Mortal Coil is a Legendary Flail for the Crusader in Diablo III, obtained via purchase of Reaper of Souls Collector's Edition (or Digital Deluxe edition) as part of the Arma Mortis pack. It can only be used for Transmogrification.
"I discovered I might bind the darkness of the human soul into metal armaments as they are being forged. Evil intent literally roils off the metal, and I've never wielded anything so wickedly sharp." — Author Unknown, Armis Mortem