Bane of the Living

Oracle Text

Morph XBB (You may cast this card face down as a 2/2 creature for 3. Turn it face up any time for its morph cost.)

When Bane of the Living is turned face up, all creatures get -X/-X until end of turn.

Card Rulings

10/4/2004 The trigger occurs when you use the Morph ability to turn the card face up, or when an effect turns it face up. It will not trigger on being revealed or on leaving the battlefield.
10/4/2004 The X in the ability has the same value as the X paid in the Morph ability. This is pretty easy to derive since there is no other source of X.