Dirgur Nemesis

• WOTC Official Reminder Textofficial rules
  Defender — This creature can't attack.

• WWizards of the Coast2015-02-25
  Official Ruling
  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.
• WWizards of the Coast2015-02-25
  Official Ruling
  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.
• WWizards of the Coast2015-02-25
  Official Ruling
  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.
• WWizards of the Coast2015-02-25
  Official Ruling
  If an attacking face-down Dirgur Nemesis is turned face up, it will continue to be attacking even though it will have defender.

Rulings via Scryfall