Voidmage Apprentice
Creature — Human Wizard
Morph 
(You may cast this card face down as a 2/2 creature for 
. Turn it face up any time for its morph cost.)
When this creature is turned face up, counter target spell.
Jim Nelson
Legions (LGN)
• #54Rulings
If a spell with split second is on the stack, you can still respond by turning this creature face up and targeting that spell with the trigger. This is because split second only stops players from casting spells or activating abilities, while turning a creature face up is a special action.
(wotc • 2013-04-15)
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.
(wotc • 2004-10-04)
Rulings
If a spell with split second is on the stack, you can still respond by turning this creature face up and targeting that spell with the trigger. This is because split second only stops players from casting spells or activating abilities, while turning a creature face up is a special action.
(wotc • 2013-04-15)
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.
(wotc • 2004-10-04)