Morbid — When Morkrut Banshee enters the battlefield, if a creature died this turn, target creature gets -4/-4 until end of turn.
"Let go your grudges, or risk wandering the bogs forever in a murderous rage." —Hildin, priest of Avacyn
Official Rulings for Morkrut Banshee
9/22/2011 : The morbid ability is mandatory. If there are no other creatures on the battlefield when the ability triggers (and a creature died this turn), the ability must target Morkrut Banshee itself.