Qarsi Deceiver
Creature — Snake Wizard
: Add 
. Spend this mana only to cast a face-down creature spell, pay a mana cost to turn a manifested creature face up, or pay a morph cost. (A megamorph cost is a morph cost.)
Illustrated by
Dragons of Tarkir#71UncommonEnglishLegality
Rulings
The mana produced by Qarsi Deceiver can’t be used to cast a spell that instructs you to manifest cards.
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.
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.
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.