Aphetto Exterminator

Morph {3}{B} (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 this creature is turned face up, target creature gets -3/-3 until end of turn.

The List

Legions

Morph {3}{B} (You may play this face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
When Aphetto Exterminator is turned face up, target creature gets -3/-3 until end of turn.

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.