Hidden Dragonslayer
Lifelink Megamorph {2}{W} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its megamorph cost and put a +1/+1 counter on it.) When this creature is turned face up, destroy target creature with power 4 or greater an opponent controls.
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Official rulings (3) · Scryfall
Turning a face-down creature with megamorph face up and putting a +1/+1 counter on it is a special action. It doesn't use the stack and can't be responded to.
Megamorph is a variant of the morph ability. You can find more information on morph on cards with morph from the Khans of Tarkir set.
If a face-down creature with megamorph is turned face up some other way (for example, if you manifest a card with megamorph and then pay its mana cost to turn it face up), you won't put a +1/+1 counter on it.